###原因
${file}はフルパスに置き換えられることが原因でした。"${workspaceRoot}/${file}"ではなく"${file}"と記載すればOKでした。
@nodai2h_ITC さん、ご指摘ありがとうございました。

###実行環境
macOS (version: 11.1)
VSCode (version: 1.51.1)
Ruby (version: 2.7.2)

###対処法
launch.json内のprogramの指定を、"${workspaceRoot}/${file}"ではなく"${file}"に変更すればOK

launch.json

{
    // Use IntelliSense to learn about possible attributes.
    // Hover to view descriptions of existing attributes.
    // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
    "version": "0.2.0",
    "configurations": [
        {
            "name": "Debug Local File",
            "type": "Ruby",
            "request": "launch",
            //"program": "${workspaceRoot}/${file}",
            "program": "${file}",
            "useBundler": true
        }
    ]
}

###エラー内容

Uncaught exception: cannot load such file -- /Users/kzyonzw/works/ruby/Users/kzyonzw/works/ruby/main.rb
	/Users/kzyonzw/.rbenv/versions/2.7.2/bin/rdebug-ide:23:in `load'
	/Users/kzyonzw/.rbenv/versions/2.7.2/bin/rdebug-ide:23:in `<top (required)>'
	/Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/lib/bundler/cli/exec.rb:63:in `load'
	/Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/lib/bundler/cli/exec.rb:63:in `kernel_load'
	/Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/lib/bundler/cli/exec.rb:28:in `run'
	/Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/lib/bundler/cli.rb:494:in `exec'
	/Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/lib/bundler/vendor/thor/lib/thor/command.rb:27:in `run'
	/Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/lib/bundler/vendor/thor/lib/thor/invocation.rb:127:in `invoke_command'
	/Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/lib/bundler/vendor/thor/lib/thor.rb:392:in `dispatch'
	/Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/lib/bundler/cli.rb:30:in `dispatch'
	/Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/lib/bundler/vendor/thor/lib/thor/base.rb:485:in `start'
	/Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/lib/bundler/cli.rb:24:in `start'
	/Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/exe/bundle:49:in `block in <top (required)>'
	/Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/lib/bundler/friendly_errors.rb:130:in `with_friendly_errors'
	/Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/exe/bundle:37:in `<top (required)>'
	/Users/kzyonzw/.rbenv/versions/2.7.2/bin/bundle:23:in `load'
	/Users/kzyonzw/.rbenv/versions/2.7.2/bin/bundle:23:in `<main>'
bundler: failed to load command: rdebug-ide (/Users/kzyonzw/.rbenv/versions/2.7.2/bin/rdebug-ide)
/Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/ruby-debug-ide-0.7.2/lib/ruby-debug-ide.rb:106:in `debug_load': cannot load such file -- /Users/kzyonzw/works/ruby/Users/kzyonzw/works/ruby/main.rb (LoadError)
	from /Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/ruby-debug-ide-0.7.2/lib/ruby-debug-ide.rb:106:in `debug_program'
	from /Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/ruby-debug-ide-0.7.2/bin/rdebug-ide:193:in `<top (required)>'
	from /Users/kzyonzw/.rbenv/versions/2.7.2/bin/rdebug-ide:23:in `load'
	from /Users/kzyonzw/.rbenv/versions/2.7.2/bin/rdebug-ide:23:in `<top (required)>'
	from /Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/lib/bundler/cli/exec.rb:63:in `load'
	from /Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/lib/bundler/cli/exec.rb:63:in `kernel_load'
	from /Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/lib/bundler/cli/exec.rb:28:in `run'
	from /Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/lib/bundler/cli.rb:494:in `exec'
	from /Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/lib/bundler/vendor/thor/lib/thor/command.rb:27:in `run'
	from /Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/lib/bundler/vendor/thor/lib/thor/invocation.rb:127:in `invoke_command'
	from /Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/lib/bundler/vendor/thor/lib/thor.rb:392:in `dispatch'
	from /Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/lib/bundler/cli.rb:30:in `dispatch'
	from /Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/lib/bundler/vendor/thor/lib/thor/base.rb:485:in `start'
	from /Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/lib/bundler/cli.rb:24:in `start'
	from /Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/exe/bundle:49:in `block in <top (required)>'
	from /Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/lib/bundler/friendly_errors.rb:130:in `with_friendly_errors'
	from /Users/kzyonzw/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/bundler-2.2.5/exe/bundle:37:in `<top (required)>'
	from /Users/kzyonzw/.rbenv/versions/2.7.2/bin/bundle:23:in `load'
	from /Users/kzyonzw/.rbenv/versions/2.7.2/bin/bundle:23:in `<main>'