GitHub Actions fail, probably due to new Ubuntu 24.04

[KrisThielemans]

GHA has transitioned to Ubuntu 24.04, see actions/runner-images#10636.

#1549 sees

/usr/bin/ld: warning: libssl.so.1.1, needed by /home/runner/work/STIR/STIR/root/lib/libNet.so.6.28.12, not found (try using -rpath or -rpath-link)

This is probably due to ccache where we use the ubuntu-latest image

STIR/.github/workflows/build-test.yml

Line 41 in a008e73

- os: ubuntu-latest

and therefore a key that is not specific to the Ubuntu version

STIR/.github/workflows/build-test.yml

Line 285 in a008e73

key: ${{ matrix.os }}-${{ matrix.compiler }}${{ matrix.compiler_version }}-cuda${{ matrix.cuda_version }}-${{ matrix.BUILD_TYPE }}