cmd/go: disable tool version check during bootstrap

The check that the go tool version matched the go compiler version was
too aggressive and didn't cover the bootstrapping case with make.bash.

We never noticed because we never had a VERSION file in place.

Repro:

$ echo "go1.9beta1" > $GOROOT/VERSION
$ cd $GOROOT/src
$ ./make.bash

No test, because x/build/cmd/release catches it.

Updates #19064
Fixes #20674

Change-Id: Ibdd7a92377f4cc77d71ed548f02d48bde6550f67
Reviewed-on: https://go-review.googlesource.com/45778Reviewed-by: Chris Broadfoot <cbro@golang.org>

src/cmd/go/internal/work/build.go

@@ -396,7 +396,7 @@ func BuildModeInit() {
 			cfg.BuildContext.InstallSuffix += codegenArg[1:]
 		}
 	}
-	if strings.HasPrefix(runtimeVersion, "go1") {
+	if strings.HasPrefix(runtimeVersion, "go1") && !strings.Contains(os.Args[0], "go_bootstrap") {
 		buildGcflags = append(buildGcflags, "-goversion", runtimeVersion)
 	}
 }