gitlab-ci: port gradle-release-checksums.py from fdroidserver

Hans-Christoph Steinerrequested to merge
test into master

./gradle-release-checksums.py used to run whenever something was merged into fdroid/fdroidserver master. If nothing was merged in a while, then this wouldn't catch new versions in a timely way. Now, this runs immediately after the new entry is added to the transparency log, so it should be quite timely.

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