CI: kconfig: allow failure when updating kconfigcheck.toml (MR 5836)

When enabling new options in kconfigcheck, it's likely that some kernels
in pmaports will fail, but often undesirable to rebuild all the kernels
particularly when the options being enabled are for quality of life.

Adjust the rules for the kconfig check job in CI so that it will fail
with a warning in this case, but still fail with an error when an
updated kernel package doesn't pass.

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>
[ci:skip-build]: already built successfully in CI

.gitlab-ci.yml

@@ -108,6 +108,9 @@ kernel-kconfig:
     - if: *mr_rule
       changes:
         - kconfigcheck.toml
+      allow_failure: true
+    - if: *mr_rule
+      changes:
         - device/*/linux-*/config-*
   script:
     - .ci/lib/gitlab_prepare_ci.sh