cerbero ci: Refactor cerbero_before_script

cerbero ci: Refactor cerbero_before_script

We were doing a few things suboptimally:

1. Making a symlink from the current directory to
   ../../gstreamer/cerbero so that build-tools kept working, which
   should not be necessary since cerbero relocates all binaries inside
   the cerbero homedir after extracting the cache.

2. Hard-coding the cerbero builddir when building on Windows because
   `pwd` was outputting an MSYS-style path which Python can't read

3. rsync-ing all the contents of $CERBERO_HOST_DIR, which is
   bad because we should be building on top of the artifact cache.
   The only thing we want from $CERBERO_HOST_DIR is cerbero-sources
   (the network cache), and we can setup the toolchain during
   bootstrap with the new --system and --toolchains arguments which
   were split out of --system-only.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-ci/-/merge_requests/350>