feat: Get the branch name using the pull request response

[srz-zumix]

Description:

Changed to get branch name from pull request response.
This allows labeling with head-branch/base-branch rules outside of pull_request events.

e.g.

on:
  workflow_dispatch:
    inputs:
      pr-number:

jobs:
  labeler:
    permissions:
      contents: read
      pull-requests: write
    runs-on: ubuntu-latest
    steps:
    - uses: actions/labeler@v5
      with:
        pr-number: ${{ inputs.pr-number }}

Related issue:

#810

Check list:

• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.