tools/toolbox: Skip install step with --dist

tools/toolbox: Skip install step with --dist

The dist command only needs an up-to-date build directory, it
does not require the project to be installed first.

While the install step is generally quick, leaving the container
file system alone can save a rebuild after the dist is done
(for instance when testing a branch, then temporarily switching
to the release branch for dist).

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3497>