explicit CI runner version + opt-in -Werror

[Un1q32]

If github were to update ubuntu-latest to 26.04, it could break CI. For example, it would update the compiler, which could introduce new warnings, or the new versions of shell utilities could be different in an incompatible way.

Also, -Werror should only be used in CI for many reasons like:

• If the project is abandoned, and someone tries to compile it years after it stops being maintained, it will probably fail because GCC will add new warnings.
• The build might fail if someone uses a slightly older compiler with different warning semantics that nobody has tested.
• When doing active development, it can be annoying to always have to fix all the small things like unused function or parameter warns.