CI: Fix CI trying to rebuild gnome-desktop, not actually building xdg-desktop-portal-gnome

I noticed that after building the gnome-desktop dependency, the pipeline doesn't go back to the source directory:

...
Installing /builds/knuxify/xdg-desktop-portal-gnome/gnome-desktop/_build/meson-private/gnome-desktop-3.0.pc to /usr/lib64/pkgconfig
Running custom install script '/usr/bin/meson --internal gettext install --subdir=po --localedir=share/locale --pkgname=gnome-desktop-3.0'
$ meson . _build
Directory already configured.
Just run your build command (e.g. ninja) and Meson will regenerate as necessary.
If ninja fails, run "ninja reconfigure" or "meson --reconfigure"
to force Meson to regenerate.
If build failures persist, run "meson setup --wipe" to rebuild from scratch
using the same options as passed when configuring the build.
To change option values, run "meson configure" instead.
$ ninja -C _build
ninja: Entering directory `_build'
ninja: no work to do.
$ ninja -C _build install
ninja: Entering directory `_build'
[0/1] Installing files.
Installing libgnome-desktop/libgnome-desktop-4.so.1.0.0 to /usr/lib64
...

(see the last line - it seems to be re-installing all the files for libgnome-desktop.)

This MR adds a cd .. to the gnome-desktop build step for both OS-es.