ci: Do not run tests for CiTest builds

Given that they are built in release mode, they already take longer to
compile.
If we run cargo test, they will be compiled again in debug mode and are
very likely to reach the timeout.

.gitlab-ci/build.yml

@@ -33,6 +33,7 @@ build@aarch64:
     APP_ID: "org.gnome.Fractal.CiTest"
     RUNTIME_REPO: "https://flathub.org/repo/flathub.flatpakrepo"
     BUNDLE: "${APP_ID}.flatpak"
+    RUN_TESTS: 0
 
 build-stable@x86_64:
   extends: