ci: Include flatpak template as a project rather than URL

It might get rid of the 429 errors.

.gitlab-ci/build.yml

@@ -1,6 +1,8 @@
 # Build the Flatpak
 
-include: 'https://gitlab.gnome.org/GNOME/citemplates/-/raw/master/flatpak/flatpak_ci_initiative.yml'
+include:
+  - project: "GNOME/citemplates"
+    file: "flatpak/flatpak_ci_initiative.yml"
 
 variables:
   RUNTIME_REPO: "https://nightly.gnome.org/gnome-nightly.flatpakrepo"

.gitlab-ci/publish_nightly.yml

@@ -1,6 +1,8 @@
 # Publish the nightly (Devel) version
 
-include: 'https://gitlab.gnome.org/GNOME/citemplates/-/raw/master/flatpak/flatpak_ci_initiative.yml'
+include:
+  - project: "GNOME/citemplates"
+    file: "flatpak/flatpak_ci_initiative.yml"
 
 publish_nightly@x86_64:
   extends: .publish_nightly