This lets distributors share a system copy of bubblewrap (>= 0.1.0)
between Flatpak and any other projects that benefit from it, if they are
careful to keep new versions in sync. The default is still to use the
bundled submodule, ensuring compatibility and simplifying dependencies.
Enable $PATH search everywhere that runs bwrap, so that $BWRAP doesn't
necessarily need to be a fully-qualified path.
Signed-off-by: Simon McVittie <smcv@debian.org>