dev/go-install.sh: only delete tmpdir if it exists
Created by: mrnugget
I've run into this multiple times:
$ ./enterprise/dev/start.sh
[...]
12:50:41 watch | rm: /var/folders/86/23tk7bxn04vgj7slgj_qdj6m0000gn/T/src-binaries.XXXXXXXX.bcuIH78v/*: No such file or directory
12:50:41 watch | Terminating watch
I don't know which condition can trigger it, but it doesn't seem wrong to be safe here.