abuild-fetch: try to work around an ESTALE error which occurs on NFS

This is because of the following race condition case:

  A                               B
                                |
  lockfd = open(lockfile, ...)  |
                                | unlink(lockfile)
  lockf(lockfd, F_LOCK, 0)      |

According to [1], to recover from an ESTALE error, an application must
close the file or directory where the error occurred, and reopen it so
the NFS client can resolve the pathname again and retrieve the new file
handle.

[1] https://nfs.sourceforge.net/#faq_a10