Pass fastlane metadata from the build step to the signing step

This complements the move of the fastlane metadata generation to ECM: frameworks/extra-cmake-modules!44 (merged)

The data isn't integrated yet into the F-Droid repository, but this should allow us to verify we produce correct data for all repositories already.

Note: this is done blindly lacking a local Jenkins test setup to verify this, so please review extra carefully :)