Enable signing of the Craft cache for qt6-dev
In principle, signing all binaries in the package stage of Craft works without signed cache (https://invent.kde.org/sdk/kirigami-gallery/-/jobs/1536514/artifacts/external_file/.logs/kde/applications/kirigami-gallery/kirigami-gallery.log), but it's a waste of bandwidth and CPU time to sign the same binaries again and again. For kirigami-gallery 164 binaries were signed which took 156 seconds in total. (Signing the installer exe took about 31 seconds in total.)
Let's try if we can sign the cache without making further modifications to Craft. The cache job builds about 230 projects and signing with the service takes at least 6 seconds (because of the 5 second polling interval of client and service). Since all projects will be signed individually, in total, the signing will take between (230*6=) 23 minutes and (230*30=) 115 minutes.
Is it okay to test this with the qt6-dev branch/cache? How do we test a full rebuild + signing of the cache?