nixos/systemd: provision a systemd-coredump user

systemd-coredump tries to drop privileges to a systemd-coredump user if
present (and falls back to the root user if it's not available).

Create that user, and recycle uid 151 for it. We don't really care about
the gid.

Fixes https://github.com/NixOS/nixpkgs/issues/120803.

nixos/modules/misc/ids.nix

@@ -187,6 +187,7 @@ in
       #seeks = 148; # removed 2020-06-21
       prosody = 149;
       i2pd = 150;
+      systemd-coredump = 151;
       systemd-network = 152;
       systemd-resolve = 153;
       systemd-timesync = 154;

nixos/modules/system/boot/systemd.nix

@@ -1053,6 +1053,7 @@ in
 
     services.dbus.enable = true;
 
+    users.users.systemd-coredump.uid = config.ids.uids.systemd-coredump;
     users.users.systemd-network.uid = config.ids.uids.systemd-network;
     users.groups.systemd-network.gid = config.ids.gids.systemd-network;
     users.users.systemd-resolve.uid = config.ids.uids.systemd-resolve;