Make `IRON_CACHE_DIR` an environment variable

[AndraBisca]

This line in iron/jit.py sets the path for the JIT cache:
IRON_CACHE_DIR = os.path.expanduser("~/.iron/cache")

It would be nice to have the ability to change this without having to modify the code directly.

[ypapadop-amd]

XDG Base Directory spec (https://specifications.freedesktop.org/basedir-spec/latest/) expects user-specific cached data to be in $XDG_CACHE_HOME or if not defined, ~/.cache. It would be nice if we respect that too.