Various CI improvements + fix wrong format check in CI

[rmalmain]

No description provided.

[tokatoka]

after this i'll change every version of these fuzzer to 0.13.2 in
#2423

[rmalmain]

ok. i'm trying to find a better way to get the version of llvm we want without relying on the github action

[rmalmain]

the improvements in this pr are:

• use of LLVM 19 by default
• LLVM version should be much easier to switch from now on
• a lot of code repetition has been deleted
• removed llvm-related github action
• fix format check
• other small things

[tokatoka]

why apt-get?

[rmalmain]

apt-get is the backend of apt, and is supposed to keep more stable over time.