src/goEnv: unset GOOS/GOARCH/GOROOT/... from tool installation env

Tool installation is to build tools that would run in the host
machine. Applying GOOS/GOARCH/GOROOT/GOFLAGS/GOENV set to build
a cross-platform project is not the right choice.

Unset them and let the `go` command figure out.

There could be other environment variables that would affect
build (GCCGO, ..), and we will consider unsetting them.

Update #628

Change-Id: I295a0ae455aa2624550a16f69686bdf191fe41fd
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/264323
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Suzy Mueller <suzmue@golang.org>

Author: hyangah

src/goEnv.ts

@@ -39,6 +39,15 @@ export function toolInstallationEnvironment(): NodeJS.Dict<string> {
 	}
 	env['GOPATH'] = toolsGopath;
 
+	// Unset env vars that would affect tool build process: 'GOROOT', 'GOOS', 'GOARCH', ...
+	// Tool installation should be done for the host OS/ARCH (GOHOSTOS/GOHOSTARCH) with
+	// the default setup.
+	delete env['GOOS'];
+	delete env['GOARCH'];
+	delete env['GOROOT'];
+	delete env['GOFLAGS'];
+	delete env['GOENV'];
+
 	return env;
 }