CI now checks that plugin caches are up to date, for example
when adding a new property to an element.
This is something pretty easy to forget, and it can be checked
in a pre-commit hook provided the cache generator generates no
unnecessart diff on the developer's machine. This is now the
case for me in core, -base and -good, and eventually all the
repositories should behave appropriately, at least on my
development machine.
The new check in the pre-commit hook is only performed when
the user is in the development environment (it checks
$MESON_BUILD_ROOT to determine that), and when the developer
has opted-in by setting `GST_CACHE_HOOK=enabled`. That is
because the hook will actually rebuild the cache with ninja,
and modify it in the source directory, that's not a behaviour
we want to enable by default.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/546>
