Supporting system Flatpak installation from org.flatpak.Builder.BaseApp

It's very likely that `org.flatpak.Builder.BaseApp` doesn't work w/ `--system` installations though it could.

Just leaving some notes here for you since a number of the same patches we do in Builder and associated projects are in here too.

While looking at https://gitlab.gnome.org/chergert/manuals/-/issues/8 today I came across all of these things that had to be fixed to get things to work where a Flatpak'd app could install to `--system` using libflatpak.

* `--filesystem=/var/tmp` seems to be required now (I don't remember this being necessary in the long past) or else the fusermount/revokefs stuff breaks. That will need to be communicated to consumers (in addition to `/var/lib/flatpak`)
* `-Dsession_helper=enabled` is necessary or Flatpak will skip trying to install using the session helper when you're inside the Flatpak
* To do the above, you'll need polkit. I still use an older version because I have patches to make that build nice
* You might want to add `--watch-bus` [here](https://github.com/flathub/org.flatpak.Builder.BaseApp/blob/branch/24.08/fusermount-wrapper.sh)
* `--system-talk-name=org.freedesktop.Flatpak.SystemHelper`