Avoid leaking file metadata (mtime/UID/GID) in rlibs through the ar crate.

[eddyb]

Fixes #889 (but do not merge until @Nehliin can confirm it works on GCP).

Both ways to test this from #889 (comment) pass cleanly now:

test result: ok. 184 passed; 0 failed; 1 ignored; 0 measured; 0 filtered out; finished in 25.09s

1999999997@a87f7a319144:~/rust-gpu$ id
uid=1999999997(1999999997) gid=1899999996(1899999996) groups=1899999996(1899999996)

$ find target -type f -name '*.rlib' -exec bash -c 'ar tv "{}" | grep -v 0/0 > /dev/null && (echo "{}:"; ar tv "{}"; echo)' ';'

(no output from that find command, i.e. no .rlib files with UIDs/GIDs embedded in them, anymore)

[Nehliin]

I can confirm this fixes the issue! Thanks!