CI: Unify pfunit build

[danielhollas]

• Use custom action to reduce repetitivness
• use $ImageOS to automatically invalidate cache when OS version changes, see CI: Use ImageOS (ubuntu24) instead of runner.os (Linux) in cache keys python/cpython#130200

[danielhollas]

Now that we don't test with GCC-7, we should be able to update pFUnit version (we are currently pinning to a specific commit due to a segfault when using GCC-7).

[JanosJiri]

Btw, isn't that solved now? @danielhollas

[danielhollas]

Kinda.

Use custom action to reduce repetitivness

This point could still be worth doing, but is low priority at the moment