cmd/coordinator: fix crash on nil global variable buildenv

This local variable was shadowing the global one.

Change-Id: If4a28de430050d884097a5474f8bb56c45c31fb1
Reviewed-on: https://go-review.googlesource.com/20958
Reviewed-by: Andrew Gerrand <adg@golang.org>

cmd/coordinator/gce.go

@@ -77,7 +77,7 @@ func initGCE() error {
 		*buildEnvName, _ = metadata.ProjectID()
 	}
 
-	buildEnv := buildenv.ByProjectID(*buildEnvName)
+	buildEnv = buildenv.ByProjectID(*buildEnvName)
 
 	// If running on GCE, override the zone and static IP, and check service account permissions.
 	if metadata.OnGCE() {