ci: make Perforce binaries executable for all users

The Perforce binaries are only made executable for the current user. On GitLab CI though we execute tests as a different user than "root", and thus these binaries may not be executable by that test user at all. This has gone unnoticed so far because those binaries are optional -- in case they don't exist we simply skip over tests requiring them. Fix the setup so that we set the executable bits for all users. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>