ci: Make test-docs job depend on sanity job

ci: Make test-docs job depend on sanity job

Without this, the test-docs job could end up waiting for manual action
after the sanity job failed, which prevented the pipeline as a whole
from having failed status.

(This means the test-docs job will no longer exist in one corner case
where it did before, when pushing directly to a non-master branch of
the main repository. That should be fine, since the docs are only
deployed from master.)

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