Revert Coverity build to use Fedora 33

The Coverity scan app can't handle GCC 11 yet, and will complain about
things inside the stdlib headers and only compile 15 files. So we need
to switch back to Fedora 33 and use GCC 10 instead.

.gitlab/coverity.yml

@@ -39,7 +39,8 @@ Coverity:
   tags:
     - coverity
   stage: build
-  image: registry.gitlab.com/kicad/kicad-ci/source_containers/master/fedora:34
+  # Use Fedora 33 because Coverity needs GCC 10 still (as of 06/16/2021)
+  image: registry.gitlab.com/kicad/kicad-ci/source_containers/master/fedora:33
   only:
     refs:
       - schedules