ci: do not mount already mounted directories

In some cases we are using an initramfs image that pre-mounts some of
the directories, so let's skip them in the next stage.

Reviewed-by: Eric Engestrom <eric@igalia.com>
Reviewed-by: David Heidelberg <david.heidelberg@collabora.com>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26552>
(cherry picked from commit d0ae58f4134bededcab45cf80dc9f194fdc1bdf3)
2 files changed