extension: customize build for tools

[rockdrilla]

This change allows one to specify build settings used to install tools.

E.g. the one may specify "GOAMD64=v3" for tools (in order to match current
machine specs) but proceed with defaults (i.e. "GOAMD64=v1")
for builds/tests globally.

[rockdrilla]

NB: this PR is merely RFC despite of code working.

[rockdrilla]

Example settings:

{
    "go.toolsManagement.buildFlags": [
        "-ldflags", "-s -w"
    ],
    "go.toolsManagement.buildTags": "grpcnotrace",
    "go.toolsManagement.envVars": {
        "CGO_ENABLED": "0",
        "GOAMD64": "v3"
    }
}

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

---

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[gopherbot]

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Patch Set 1:

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[gopherbot]

This PR (HEAD: df6c792) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/vscode-go/+/571856.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to log in to Gerrit.
• To change your code in response to feedback:
  • Push a new commit to the branch used by your GitHub PR.
  • A new "patch set" will then appear in Gerrit.
  • Respond to each comment by marking as Done in Gerrit if implemented as suggested. You can alternatively write a reply.
  • Critical: you must click the blue Reply button near the top to publish your Gerrit responses.
  • Multiple commits in the PR will be squashed by GerritBot.
• The title and description of the GitHub PR are used to construct the final commit message.
  • Edit these as needed via the GitHub web interface (not via Gerrit or git).
  • You should word wrap the PR description at ~76 characters unless you need longer lines (e.g., for tables or URLs).
• See the Sending a change via GitHub and Reviews sections of the Contribution Guide as well as the FAQ for details.