ci: appimage build: run build.sh directly instead of sub-scripts

- previously we had the CI spawn us inside a docker container as per our Dockerfile,
  and we ran make_appimage.sh inside that
- now we also need to run make_type2_runtime.sh, which builds and runs another docker container, so the old approach does not work
  - follow-up https://github.com/spesmilo/electrum/pull/10019
- looks easier to just try to run build.sh, the outer script, which does all that
  - makes the CI task more similar to what dev machines run
  - at the cost of somewhat more compute

.cirrus.yml

@@ -344,8 +344,10 @@ task:
       only_if: $CIRRUS_CRON == ""
     - trigger_type: automatic
       only_if: $CIRRUS_CRON == "nightly"
-  container:
-    dockerfile: contrib/build-linux/appimage/Dockerfile
+  compute_engine_instance:
+    image_project: cirrus-images
+    image: family/docker-builder
+    platform: linux
     cpu: 2
     memory: 2G
   pip_cache:
@@ -361,11 +363,9 @@ task:
       - cat contrib/make_libsecp256k1.sh | sha256sum
       - git ls-files -s contrib/build-linux/appimage/
   build_script:
-    - ./contrib/build-linux/appimage/make_appimage.sh
+    - ./contrib/build-linux/appimage/build.sh
   binaries_artifacts:
     path: "dist/*"
-  env:
-    CIRRUS_DOCKER_CONTEXT: contrib/build-linux/appimage
   depends_on:
     - "unittests: py3.10"