Bug#1079246: gcr4: Environment variable SSH_AUTH_SOCK is hijacked by gcr-ssh-agent

[Alessandro Astone]

Perhaps this was different at the time the bug was reported, but the 
systemd socket unit is provided by the `gcr4` binary package, and not 
the `libgcr-4-4` binary package.

The gcr4 binary package should only be installed if you want the gcr4 
ssh agent, and in that case it is intended that it tries to set itself 
as the default ssh agent provider.