Add the requirements-key parameter to the pyenv GitHub Action

[drew2a]

This PR fixes #7026 by adding the dedicated requirements-key parameter.

This parameter will be used in the restore-virtualenv@v1.2.1 action as a value on which the cache key be generated.

    - name: Restore virtual env
      uses: drew2a/restore-virtualenv@v1.2.1
      id: cache-virtualenv
      with:
        requirement_files: ${{inputs.requirements-key}}

The old parameter requirements is still be used for passing it into:

    - name: Install pip dependencies
      if: steps.cache-virtualenv.outputs.cache-hit != 'true' || inputs.invalidate-cache == 'true'
      shell: bash
      run: |
        python -m pip install --upgrade pip
        pip install -r ${{inputs.requirements}}

Therefore by default for the cache key generation, we use the pattern **/requirements*.txt which can be translated into:

requirements-build.txt
requirements-core.txt
requirements-test.txt
requirements.txt

Therefore **/requirements*.txt value suits all our actions although it is a bit redundant (by default).