x/tools/gopls: Add note to vscode doc about tags

[perrito666]

As prompted this issue it
is unclear how to setup build tags in gopls in vscode.

vscode has a configuration section specific for go build tags but it is
not honored by gopls which instead requires GOFLAGS environment variable
to be set with the -tags flag.

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[perrito666]

@googlebot I signed it!

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This PR is being closed because golang.org/cl/233018 has been merged.