CI: temporarily disable fedora-clang

The g-ir-scanner build is failing with clang, passing unknown arguments
-mcet and -fcf-protection. I can't find where they're coming from, but
it's also happening on master so it is probably a problem with Rawhide.
4 jobs for 117-fix-warnings in 16 minutes and 22 seconds (queued for 36 minutes and 21 seconds)
Status Job ID Name Coverage
  Build
passed #6768
coverage

00:11:14

passed #6769
cppcheck

00:00:25

passed #6767
fedora_gcc

00:04:42

 
  Failure
skipped #6770
on_failure