The exec wrapper lock file was opened, locked and then never used
again, assuming it would close and unlock at process exit.
However, the garbage collector could collect and run the *os.File
finalizer that closes the file prematurely, rendering the lock
ineffective.

Make the lock global so that the lock is live during the entire
execution.

(Hopefully) fix the iOS builders.

Change-Id: I62429e92042a0a49c4f1ea553fdb32b6ea53a43e
Reviewed-on: https://go-review.googlesource.com/21137Reviewed-by: David Crawshaw <crawshaw@golang.org>