Add environment variables to signal feature and version to the CLI

This PR ensures environment variables are set before any invocation of
the CLI.  Here is a list of vars that are set:

https://github.com/github/codeql-coreql-team/issues/1124#issuecomment-852463521

This ensures the CLI knows the features and versions of the containing
actions/runner.

Additionally:

- Fix the user agent so that it more closely aligns with user agent
  spec
- Refactor environment variable initialization so that it all happens in
  one place and call.
- Move Mode, getRequiredEnvParam, setMode, getMode out of actions-util
  and into util. actions-util is meant for utils only called by the
  action, not the runner.

The `prepareLocalRunEnvironment()` method is most likely deprecated and
should be removed. I originally added it because I had a way of working
where I would run the action from my local machine to test out changes,
but this was always a little flaky. So, I no longer use this way of
working. I will probably remove it soon.

src/toolcache.ts

@@ -8,8 +8,8 @@ import * as actionsToolcache from "@actions/tool-cache";
 import * as safeWhich from "@chrisgavin/safe-which";
 import * as semver from "semver";
 
-import { isActions } from "./actions-util";
 import { Logger } from "./logging";
+import { isActions } from "./util";
 
 /*
  * This file acts as an interface to the functionality of the actions toolcache.