Mk/bsd.port.mk: Export XDG_CACHE_HOME

This adds XDG_CACHE_HOME to the set of XDG_* variables exported in
CONFIGURE_ENV and MAKE_ENV.

Without XDG_CACHE_HOME in the environment some software (for ex.
https://github.com/coursier/coursier) falls back to using
/home/{user}/.cache during the build and writes outside the working
directory.

XDG_CACHE_HOME is described in the XDG Base Directory Specification:
https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html

PR: 255600
Exp-run by: antoine