ui: make it clear to present the environment setting the extension is using

[hyangah]

As discussed in #1595 and other bug reports,
VSCode Go inherits the environment variables the vscode window is launched with.
And then, go.toolsEnvVars can be used to override some of them more explicitly.
Go: Locate Configured Go Tools provide a hacky way of showing the list of env vars
but that doesn't show how the extension picked them up. That leads confusion.

Improve the UI/UX and clearly display

• what environment variables are in effect
• why (e.g. extension host's env var, go.toolsEnvVars setting, go, or the extension's manipulation)

and, ideally an interface to override them easily.

[hyangah]

Duplicate of #2049