gitlab-ci: `fdroid build` job fully handles reproducible builds
metadata/*/signatures/*
are used for reproducible builds. If those files
change, then fdroid build
should run and try to build the app with those
signature files. Also, fdroid publish
needs to run to copy those
signature files to complete the test of reproducibility. Builds that use
the Binaries:
method are already verified as part of fdroid build
.