This option allows to specify if the action shall update environment variables (default) or not.
This allows to use the setup-python action in a composite action without side effect (except downloading/installing python if version is missing).
This pull-request improves `setup-python` action to add ability to download specific version of Python on flight if it is not available by default.
**Details:**
`setup-python` action will download and install specific Python version from GitHub releases ([actions/python-versions](https://github.com/actions/python-versions/releases)) in case the version is not found in the local cache. All versions of Python available for installation are published in [actions/python-versions](https://github.com/actions/python-versions) repository.
All available versions are listed in the [version-manifest.json](https://github.com/actions/python-versions/blob/master/versions-manifest.json) file.
**Installation time:**
- Ubuntu / macOS: 10-20 seconds
- Windows: ~ 1 minute (mostly related to fact that we use MSI installer for Python on Windows)
Co-authored-by: MaksimZhukov <v-mazhuk@microsoft.com>
Co-authored-by: Konrad Pabjan <konradpabjan@github.com>
Co-authored-by: Brian Cristante <33549821+brcrista@users.noreply.github.com>
