sweet/harnesses: in go-build harness rebuild go command too

We rebuild the compiler and linker for each of the go-build runs, but
they're not happy if their versions (including the go experiment) don't
match up with the go command's version. So rebuild the go command with
the same configuration so everything matches.

Change-Id: I606db082e0dedfc3c396e731f5fb8bccd0f61a79
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/689615
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>

Author: matloob

sweet/harnesses/go-build.go

@@ -117,8 +117,8 @@ func (h GoBuild) Build(pcfg *common.Config, bcfg *common.BuildConfig) error {
 		return fmt.Errorf("error copying GOROOT: %v", err)
 	}
 	cfg.GoRoot = goroot
-	if err := cfg.GoTool().Do("", "install", "cmd/compile", "cmd/link"); err != nil {
-		return fmt.Errorf("error building cmd/compile and cmd/link: %v", err)
+	if err := cfg.GoTool().Do("", "install", "cmd/go", "cmd/compile", "cmd/link"); err != nil {
+		return fmt.Errorf("error building cmd/go, cmd/compile, and cmd/link: %v", err)
 	}
 
 	// Get the benchmarks we're going to build.