* Move the exported PATH variables into a script so
we can source it in all the wrapper scripts.
* Add a guard to only copy over the cache when the
SUBPROJECTS_CACHE_DIR variable is set, which only
happens on CI by default
* Make it possible to run the scripts with default
values for some of the variables, like WERROR and MESON_ARGS.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7591>
