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`.
fdroidserver!893
fdroidserver#891
fdroiddata!8816
