feat: Add ability to cache compilers between runs

[gnikit]

It would be extremely useful if the Action had a key option cache, similar to setup-python that could enable/disable caching the downloaded compilers and the setup.

Suggestion

    - uses: fortran-lang/setup-fortran@v1.6.1
      id: setup-fortran
      with:
        compiler: ${{ matrix.toolchain.compiler }}
        version: ${{ matrix.toolchain.version }}
        cache: true  # optional, default is false

[wpbonelli]

Some notes

• We already cache the Intel toolchain on Windows. The key currently rotates daily to avoid a broken state where the cache can neither be restored nor refreshed, backstory here. Not sure if still relevant but even if so, maybe we just warn people may need to occasionally clear it?
• Windows seems like highest priority as even GCC and LFortran installs are fairly slow (~5 and ~2.5 min respectively). Intel is usually slowest, although on Ubuntu the slowest is Nvidia. But makes sense to cache all platforms/compilers IMO