Switch to the newer base CI image

Switch to the newer base CI image

We can now spare building QtKeychain over and over shortening a bit the
build. We also update to latest gcc and clang.

Note that I didn't switch the AppImage pipeline to the newer image to
keep the binary compatibility promise required by linuxdeployqt.
Probably would make sense to do a specific image for AppImage with some
other of the cleanups I did there (in particular to avoid building
qtkeychain every time).

Signed-off-by: Kevin Ottens <kevin.ottens@nextcloud.com>