cmd/upload: use specified go version for building

There's an optional -go flag to specify Go version for compilation.
Use it in both invocations of the "go" command, not just the first.

This is a followup to CL 207420 where the -go flag was added.

Change-Id: Ic33b9acf194fb8fda582d3495e4247fec36218e0
Reviewed-on: https://go-review.googlesource.com/c/build/+/216837
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/cmd/upload/upload.go b/cmd/upload/upload.go
index b3733a3..6b8edbc 100644
--- a/cmd/upload/upload.go
+++ b/cmd/upload/upload.go
@@ -207,12 +207,13 @@
 		goos, goarch = v[0], v[1]
 	}
 
+	goBin := goCmd()
 	env := append(os.Environ(), "GOOS="+goos, "GOARCH="+goarch)
 	if *extraEnv != "" {
 		env = append(env, strings.Split(*extraEnv, ",")...)
 	}
 	env = envutil.Dedup(runtime.GOOS == "windows", env)
-	cmd := exec.Command(goCmd(),
+	cmd := exec.Command(goBin,
 		"list",
 		"--tags="+*tags,
 		"--installsuffix="+*installSuffix,
@@ -236,7 +237,7 @@
 	if *static {
 		ldflags = "-linkmode=external -extldflags '-static -pthread' " + ldflags
 	}
-	cmd = exec.Command("go",
+	cmd = exec.Command(goBin,
 		"install",
 		"--tags="+*tags,
 		"--installsuffix="+*installSuffix,