簡単なことなんですが、めちゃはまったので。。。

snake_case って文字が snake-case なります

name: snake case to kebab case smple

on:
  workflow_dispatch:
    inputs:
      input_string:
        description: 'select input string'
        required: true
        type: choice
        default: 'snake_case'
        options:
        - snake_case
        - kebab-case
          
jobs:
  CaseChange:
    runs-on: ubuntu-latest
    steps:
      - name: echo input
        run : |
          echo ${{ inputs.input_string }}
      - name: snake case to kebab case
        run: |
          echo "KebabCase=${{ inputs.input_string }}" | sed $'s/_/-/g' >> $GITHUB_ENV
      - name: use kebab case
        run: |
          echo ${{ env.KebabCase }}