Add env var check for worker runtime in StartHostAction

[Zemfm]

Adds a check to StartHostAction to use the environment variable "FUNCTIONS_WORKER_RUNTIME" if available, before displaying the Wizard Prompt.

This enables setting the runtime through environment variables as was the case in the past.

Issue describing the changes in this PR

resolves #4193

Pull request checklist

• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• I have added all required tests (Unit tests, E2E tests)

Note: I was unable to run or write unit tests and only validated manually by running the tool against a function with/without environment variables set:
https://github.com/Azure/azure-functions-core-tools/blob/v4.x/CONTRIBUTING.md#running-the-test-suite

I was able to run the tests and added a unit test. It would be helpful if the link above was updated.

[Zemfm]

@microsoft-github-policy-service agree

[Zemfm]

Any chance this can get looked at? It's been open for over a month now.,,

[liliankasem]

@Zemfm thank you for contributing. I don't see any reason why you cannot use an environment variable to set the worker runtime locally, however I wanted to do some cleanup here, and have the environment logic somewhere it makes more sense (as part of the init flow for GlobalCoreToolsSettings). Because of this, I have created a new PR that should fix this issue: #4219 and will close this PR. Let me know if you have any questions. Thanks!