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.
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.