gitlab-ci: download fdroidserver as tarball rather than git clone

This should hopefully be quicker and lower impact.

.gitlab-ci.yml

@@ -2,7 +2,9 @@ image: registry.gitlab.com/fdroid/ci-images-server:latest
 
 before_script:
   - rm -rf fdroidserver
-  - git clone --depth=1 https://gitlab.com/fdroid/fdroidserver.git
+  - mkdir fdroidserver
+  - curl --silent https://gitlab.com/fdroid/fdroidserver/repository/master/archive.tar.gz
+      | tar -xz --directory=fdroidserver --strip-components=1
   - export PATH="$PWD/fdroidserver:$PATH"
   - touch config.py