ci/farms: always compare the code against main repository

ci/farms: always compare the code against main repository

`rules:changes:compare_to` resolved firstly pushed branch pipelines,
which always evaluated `rules:changes` as true which breaks the workflow

Since we now explicitely say, that we compare against `main` repository,
GitLab can evaluate against real changes.

Fixes: 79f7882fc604 ("ci: add quirk for GitLab assuming changes is always true for scheduled runs")

Reviewed-by: Eric Engestrom <eric@igalia.com>
Signed-off-by: David Heidelberg <david.heidelberg@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24305>