Description:
Add an option not to add/update environment variables.

Justification:
When using actions/setup-python, it will update or add PATH, PythonLocation, PKG_CONFIG environment variables.
If one is to use actions/setup-python inside a composite action, those environment variables update might conflict with a user workflow.

e.g.

- uses: actions/setup-python@v3
  with:
    python-version: "3.7"
- uses: some/composite-action@v1 # Runs a different setup-python internally with python-version: "3.10"
- run: python --version  # this will show python 3.10 while the user expects 3.7

With this proposal, some/composite-action could use:

- uses: actions/setup-python@v3
  id: cp310
  with:
    python-version: "3.10"
    no-environment-update: true
- run: ${{ steps.cp310.outputs.python-path }} --version  # this will show python 3.10 in the composite action

The user workflow will now print python 3.7 as expected.

Are you willing to submit a PR?
Yes: #411