logrotate/systemd: add 'minsize = 1M' to wtmp/btmp rotation

align with upstream logrotate which added the minsize rule at some point.
This avoids needlessly rotating the files too often as brought up in
https://github.com/NixOS/nixpkgs/pull/159187#issuecomment-1052426774

nixos/modules/system/boot/systemd.nix

@@ -1224,6 +1224,7 @@ in
         keep = 1;
         extraConfig = ''
           create 0660 root ${config.users.groups.utmp.name}
+          minsize 1M
         '';
       };
       "/var/log/wtmp" = mapAttrs (_: mkDefault) {
@@ -1231,6 +1232,7 @@ in
         keep = 1;
         extraConfig = ''
           create 0664 root ${config.users.groups.utmp.name}
+          minsize 1M
         '';
       };
     };

nixos/tests/logrotate.nix

@@ -19,7 +19,8 @@ import ./make-test-python.nix ({ pkgs, ...} : rec {
 
               # wtmp is present in default config.
               "rm -f /var/log/wtmp*",
-              "echo test > /var/log/wtmp",
+              # we need to give it at least 1MB
+              "dd if=/dev/zero of=/var/log/wtmp bs=2M count=1",
 
               # move into the future and rotate
               "date -s 'now + 1 month + 1 day'",