TUN-6869: Fix Makefile complaining about missing GO packages

cloudflared's Makefile uses `shell go env GOOS` to determine the
LOCAL_OS regardless of it being provided. We therefore need pinned_go as
a dependency to run docker-generate-versions.

cfsetup.yaml

@@ -81,6 +81,7 @@ stretch: &stretch
   generate-versions-file:
     build_dir: *build_dir
     builddeps: 
+      - *pinned_go
       - build-essential
     post-cache:
       - make generate-docker-version