gitlab-ci: rule anchor for experimental devices as manual in MRs

gitlab-ci: rule anchor for experimental devices as manual in MRs

We want to give developers the option to run their jobs on devices
that are still being stabilized in the CI infrastructure.

These jobs should be optional and not prevent merging from happening.

The is-forked-branch-or-pre-merge anchor was not being used anywhere,
so it was changed to is-forked-branch-or-pre-merge-not-for-marge to
create this new rule.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10405>