[GHA] force ubuntu-24.04 and update compilers

[KrisThielemans]

Using ubuntu-latest as part of the "key" for ccache means we don't catch changes in the VM.

Taking the opportunity to test gcc-14.

Fixes #1550

[KrisThielemans]

CUDA install failure with gcc-14

/usr/bin/sudo /home/runner/work/STIR/STIR/cuda_installer-linux-6.8.0-1017-azure-12.1.0/cuda_installer-linux-6.8.0-1017-azure_12.1.0.run --silent --toolkit
 Failed to verify gcc version. See log at /var/log/cuda-installer.log for details.

[KrisThielemans]

ROOT linking failure is likely due to usage old binary. should update to one of
https://root.cern/download/root_v6.32.02.Linux-ubuntu22.04-x86_64-gcc11.4.tar.gz
https://root.cern/download/root_v6.32.00.Linux-ubuntu24.04-x86_64-gcc13.2.tar.gz

[KrisThielemans]

https://github.com/UCL/STIR/actions/runs/12276026037/job/34252343981#step:9:117

CMake Error in root/cmake/ROOTConfig.cmake:
  cmake_policy PUSH without matching POP
Call Stack (most recent call first):
  src/cmake/FindCERN_ROOT.cmake:56 (find_package)
  CMakeLists.txt:121 (find_package)

The file looks fine though: https://github.com/root-project/root/blob/v6-34-00/cmake/scripts/ROOTConfig.cmake.in

[KrisThielemans]

The confusing message was caused by the suppression of any useful output by doing find_package(ROOT version QUIET) in FindCERN_ROOT.cmake.

Installing vdt solves the CMake configure problem. But then we had

/usr/bin/ld: warning: libtbb.so.12, needed by /home/runner/work/STIR/STIR/root/lib/libImt.so, not found (try using -rpath or -rpath-link)

Checking https://root.cern/install/dependencies/ shows we need tbb as well

[KrisThielemans]

success. cleaning-up now