ci_run_n_monitor: add ability to specify the pipeline to use, instead of auto-detecti...

ci_run_n_monitor: add ability to specify the pipeline to use, instead of auto-detecting it

The auto-detection code currently looks for a repo called "mesa" in the
current user's fork (ie. the user providing the api token), which is great for
the common use case, but sometimes needs to be able to be overridden, such as
when running a pipeline in another fork than one's own, when working with
someone else in their fork.

Signed-off-by: Eric Engestrom <eric@igalia.com>
Acked-by: David Heidelberg <david.heidelberg@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23230>