ci: use base fedora image for the build

The gnome-runtime-images are tailored towards building flatpak
apps, and not really suitable for simple 'host' build jobs
like this one.

Close gnome-runtime-images#4

.gitlab-ci.yml

-image: registry.gitlab.gnome.org/gnome/gnome-runtime-images/base:latest
-
 build:
+  image: 'registry.fedoraproject.org/fedora:latest'
   script:
-    - dnf install -y libappstream-glib-devel gettext
+    - dnf install -y git-core python3-pip meson ninja-build libappstream-glib-devel gettext
     - pip3 install meson ninja -r requirements.txt
     - meson build
     - ninja -C build install