ci: check that kernels have pmb:cross-native enabled (MR 1247) (MR 1529)

follow up to
https://gitlab.com/postmarketOS/pmbootstrap/-/issues/1910,
https://gitlab.com/postmarketOS/pmaports/-/merge_requests/1169 and
https://gitlab.com/postmarketOS/pmaports/-/merge_requests/1187.

Fixing the repo is good, but keeping it clean is better. Ensuring it
before merge will avoid yet another fixup commit.

(cherry picked from commit 307f4f19)