ci: Move sanity stage to the beginning of the pipeline

ci: Move sanity stage to the beginning of the pipeline

This is possible now that it uses the external ci-fairy docker image.

This allows dropping the "check mr" job from needs: of other jobs, the
container stage jobs will only become available once the sanity stage
has passed.

This also allows simplifying the "check mr" job rules and script, since
the job only needs to exist in pre-merge pipelines for MRs anymore.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7676>