keep Release.gpg on untrusted to trusted IMS-Hit

keep Release.gpg on untrusted to trusted IMS-Hit

A user relying on the deprecated behaviour of apt-get to accept a source
with an unknown pubkey to install a package containing the key expects
that the following 'apt-get update' causes the source to be considered
as trusted, but in case the source hadn't changed in the meantime this
wasn't happening: The source kept being untrusted until the Release file
was changed.

This only effects sources not using InRelease and only apt-get, the apt
binary downright refuses this course of actions, but it is a common way
of adding external sources.

Closes: 838779