Set environment variables for go tools

[bancek]

This will let users set env variables for go build (e.g. CGO_CFLAGS, CGO_LDFLAGS and LD_LIBRARY_PATH).

[msftclas]

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[ramya-rao-a]

@bancek Why only build/lint/vet? Wouldn't other tools/features benefit from this as well?

If yes, then it would make more sense to update process.env with these env variables once during extension activations, and it will get used everywhere.

@hickeng This can help your GOOS scenario also I believe. #632

[hickeng]

@ramya-rao-a Yes! Looking forward to the merge.

[bancek]

@ramya-rao-a env variables are mostly needed for building.

I don't think it's a good idea to change process.env because it would effect other extensions and especially when config can be changed while VS Code is running.

[ramya-rao-a]

@bancek See #632 (comment) for scenario where the env vars need to be used by some of the Go tools that we use the Go extension.

About your concerns on changes to process.env affecting other extensions:

VS Code runs each extension in a separate process, and so any changes we do to the process.env is reflected only in the operations carried out by the Go extension.

See https://code.visualstudio.com/docs/extensionAPI/patterns-and-principles#_stability-extension-isolation

Edit: I was wrong, VS Code runs all extensions in the same process which is separate from the process used by the core VS Code.

Will merge this PR.