gitlab-ci: cache downloaded things between builds

This stores the Android SDK tarball and gradle caches between builds
to speed things up.  Since the unpacked SDK gets unpacked very time,
updating the version is just a matter of changing the variable.  Since
only the gradle caches are stored, i.e. the jars and gradle binaries,
updates there will only add more files to the cache.

.gitlab-ci.yml

 image: mvdan/fdroid-ci:latest
 
+cache:
+  paths:
+    - .gradle/wrapper
+    - .gradle/caches
+
 test:
   script:
+    - export GRADLE_USER_HOME=$PWD/.gradle
     - cd F-Droid
     - ./tools/langs-list-check.py
     - ./tools/check-string-format.py