ci: Use mold linker for Linux builds (#2847)

* The default linker which happens to be BFD in Ubuntu 24.04 does not support Clang's ThinLTO which CMAKE_INTERPROCEDURAL_OPTIMIZATION_RELEASE=ON tries to enable.
* Using mold linker fixes this and reduces build time a bit.
* For consistency reasons we enable mold linker for GCC builds aswell.