nixos/filesystems: condition mount-pstore.service on unmounted /sys/fs/pstore

For unknown reasons, switching to a system that first introduces this
service has it fail with /sys/fs/pstore already having been mounted.

nixos/modules/tasks/filesystems.nix

@@ -321,6 +321,7 @@ in
             RemainAfterExit = true;
           };
           unitConfig = {
+            ConditionPathIsMountPoint = "!/sys/fs/pstore";
             ConditionVirtualization = "!container";
             DefaultDependencies = false; # needed to prevent a cycle
           };