Handling Functions Runtime Environment env var when set by user

[brandonh-msft]

with accompanying test
Fixes #2465

Issue describing the changes in this PR

resolves #2465

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
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• I have added all required tests (Unit tests, E2E tests)

[liliankasem]

/azp run

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Azure Pipelines successfully started running 1 pipeline(s).

[liliankasem]

My bad, I just noticed that you're actually running the CLI with this test. So it's not a unit test but would need to use the E2E test framework we have. Here is an example of how to write one of those instead:

azure-functions-core-tools/test/Cli/Func.E2ETests/Commands/FuncStart/MissingConfigTests.cs

Lines 50 to 83 in e963a76

	public async Task Start_LanguageWorker_InvalidFunctionJson_FailsWithExpectedError()
	{
	var port = ProcessHelper.GetAvailablePort();
	var functionName = "HttpTriggerJS";
	var testName = nameof(Start_LanguageWorker_InvalidFunctionJson_FailsWithExpectedError);
	// Initialize Node.js function app using retry helper
	await FuncInitWithRetryAsync(testName, [".", "--worker-runtime", "node", "-m", "v3"]);
	// Add HTTP trigger using retry helper
	await FuncNewWithRetryAsync(testName, [".", "--template", "Httptrigger", "--name", functionName, "--language", "node"], workerRuntime: "node");
	// Modify function.json to include an invalid binding type
	var filePath = Path.Combine(WorkingDirectory, functionName, "function.json");
	var functionJson = await File.ReadAllTextAsync(filePath);
	functionJson = functionJson.Replace("\"type\": \"http\"", "\"type\": \"http2\"");
	await File.WriteAllTextAsync(filePath, functionJson);
	// Call func start
	var funcStartCommand = new FuncStartCommand(FuncPath, testName, Log);
	funcStartCommand.ProcessStartedHandler = async (process) =>
	{
	await ProcessHelper.ProcessStartedHandlerHelper(port, process, funcStartCommand.FileWriter ?? throw new ArgumentNullException(nameof(funcStartCommand.FileWriter)));
	};
	var result = funcStartCommand
	.WithWorkingDirectory(WorkingDirectory)
	.WithEnvironmentVariable(Common.Constants.FunctionsWorkerRuntime, "node")
	.Execute(["--port", port.ToString(), "--verbose"]);
	// Validate error message
	result.Should().HaveStdOutContaining("The binding type(s) 'http2' were not found in the configured extension bundle. Please ensure the type is correct and the correct version of extension bundle is configured.");
	}

I believe there is a doc too