debian: Update to 7.6

(cherry picked from commit 1245ca3ff7b2805e7c0c09ca7e7fea7ac8bfcc5a)
Don't restart systemd-remount-fs

It's only needed during early boot (in fact, it's probably not needed
at all on NixOS). Restarting it is expensive because it does a sync()
of the root file system.

(cherry picked from commit 3b2609deec84ec71ced729f128aa9d549dcfa4f6)