Revert "Updated GitLab CI to use docker image built by GitHub Actions on ghcr.io"

This reverts commit 9c357e1f

.gitlab-ci.yml

 build:
   stage: build
   image:
-    name: ghcr.io/sdc0/manjaro-qt6-ffmpeg
+    name: sdc0/manjaro-qt6-ffmpeg
     entrypoint: ["/bin/sh", "-c"]
   script:
     - mkdir build
@@ -13,4 +13,4 @@ build:
       - build/AVQt/libAVQt_static.a
       - build/AVQt/libAVQt.so
       - AVQt/
-    expire_in: 1 week
\ No newline at end of file
+    expire_in: 1 week