Dynamic parallelism for a triggerable GHA workflow

parallel.yml

name: Dynamic parallelism
run-name: Dynamic parallelism ${{ github.ref_name }} with ${{ toJson(inputs) }}
on:
  workflow_dispatch:
    inputs:
      parallelism:
        description: How many jobs to run in parallel
        required: true
        type: string
        default: '1'

jobs:
  init:
    name: Init
    runs-on: ubuntu-latest
    timeout-minutes: 15
    outputs:
      list: ${{ steps.build.outputs.list }}
    steps:
      - name: ⬇️ Checkout repository
        uses: actions/checkout@v4
      - name: 🏗️ Build matrix
        id: build
        run: echo "list=`node -e 'process.stdout.write(JSON.stringify(new Array(${{ inputs.parallelism }}).fill(1).map((value, index) => index + 1)))'`" >> $GITHUB_OUTPUT

  dynamic:
    name: Dynamic
    needs: [init]
    timeout-minutes: 15
    runs-on: ubuntu-latest
    strategy:
      fail-fast: false
      matrix:
        index: ${{ fromJson(needs.init.outputs.list) }}
    steps:
      - name: ⬇️ Checkout repository
        uses: actions/checkout@v4