chore: don't build beta if touching flatpaks (#1134)

Just to align with the other build workflows

Author: inffy

.github/workflows/build-image-beta.yml

@@ -7,6 +7,7 @@ on:
       - testing
     paths-ignore:
       - "**.md"
+      - "flatpaks/**"
       - "system_files/shared/usr/share/ublue-os/bazaar/config.yaml"
       - "**.Brewfile"
       - "just/*.just"