Enable to override commit sha on github action

[nobuhikosawai]

This PR enables to override SHA on GitHub Action. This PR is to complement #713.

On the issue_comment event, github.sha always refers to the default branch. Therefore, if you want to trigger a chromatic action on a specific PR associated with a particular PR comment, an additional process to fetch both the branch name and sha is required. Currently, you can override the branch name by providing the branchName option, but the sha cannot be overridden. This PR enables it.

The following example shows a way to override a sha with this PR.

on: issue_comment
permissions: read-all
jobs:
  publish_chromatic:
    name: Publish to Chromatic
    runs-on: ubuntu-latest
    steps:
      - id: get-branch
        run: |
          input_string=$(gh pr view 8 --repo nobuhikosawai/ga-test --json headRefOid --json headRefName -q '"branch=\(.headRefName) sha=\(.headRefOid)"')
          branch=$(echo "$input_string" | awk -F ' ' '{split($1, a, "="); split($2, b, "="); print a[2]}')
          sha=$(echo "$input_string" | awk -F ' ' '{split($1, a, "="); split($2, b, "="); print b[2]}')
          echo "branch=$branch" >> $GITHUB_OUTPUT
          echo "sha=$sha" >> $GITHUB_OUTPUT
        env:
          REPO: ${{ github.repository }}
          PR_NO: ${{ github.event.issue.number }}
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
      - uses: actions/checkout@v4
        with:
          fetch-depth: 0
          ref: ${{ steps.get-branch.outputs.branch }}
      - name: Install dependencies
        run: yarn
      - name: Publish to Chromatic
        uses: chromaui/action@v1
        with:
          projectToken: ${{ secrets.CHROMATIC_PROJECT_TOKEN }}
          branchName: ${{ steps.get-branch.outputs.branch }}
          commitSha: ${{ steps.get-branch.outputs.sha  }}

[nobuhikosawai]

@thafryer If you have some questions or need more clarifications, please do let me know.