tools/toolbox: Add --sysext option to meson-build

tools/toolbox: Add --sysext option to meson-build

When specified, the project is installed to a separate destdir
inside the container instead of the container's root filesystem.

This can be used to install several projects into a common extension
directory:

```
 $ (cd mutter; meson-build.sh --sysext) &&
   (cd gnome-shell; meson-build.sh --sysext)
```

We will later add a separate command to extract the directory
from the container and turn it into a system extension suitable
for use with systemd-sysext.

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