CI: Upgrade mozjs78 images to Fedora 33

Allows us to move back to the distro's latest version of Meson, which no
longer has the bug that considers skipped tests as failed. Also upgrades
IWYU to 0.15, the version that goes along with Clang 11.
3 jobs for fedora33-ci-images in 47 minutes and 14 seconds (queued for 1 second)
Status Job ID Name Coverage
  Prepare
passed #1004086
build-alpine-image

00:00:09

 
  Deploy
passed #1004087
fedora.mozjs78

00:43:41

passed #1004088
fedora.mozjs78-debug

00:47:04