config: don't always attempt to combine our env with the host
config: don't always attempt to combine our env with the host

For all the environment variables we set, we only want to use our
values.  Nothing from the host should be used.  Other environment
variables should still be passed through though.

Part-of: <https://gitlab.freedesktop.org/gstreamer/cerbero/-/merge_requests/526>

GStreamer/cerbero,GStreamer/cerbero,GStreamer/cerbero,nirbheek/cerbero,nirbheek/cerbero,GStreamer/cerbero,GStreamer/cerbero,nirbheek/cerbero,nirbheek/cerbero