ci: Make --destdir option in install-meson-project cumulative

ci: Make --destdir option in install-meson-project cumulative

When building a system extension for GNOME OS, any extra
dependencies are needed both in the container (for building)
and the extension's destdir (for running).

Because of that, the --destdir flag that was added in commit
8aeb6dc86 does not only install to the specified directory
as expected, but also runs a second install step without
destdir (i.e. to the system).

However that behavior is not a good fit when we extend our
existing toolbox tooling to build system extensions for
Fedora instead of GNOME OS.

To account for that, make the --destdir option cumulative
and install the project to all provided destdirs (or /
if omitted).

This gives us the flexibility to install to the system, a
different destdir, or both:

```
 $ ./install-meson-project.sh  # install to system
 $ ./install-meson-project.sh --destdir /new/dest  # install to destdir
 $ ./install-meson-project.sh --destdir /new/dest --destdir /  # both
```

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3498>