adding root_pool/nix_build to all zfs boxes

this fixes issues with file how the test intract with zfs file systems

systems/jeeves/hardware.nix

@@ -96,6 +96,11 @@ in
       fsType = "zfs";
     };
 
+    "/nix_build" = {
+      device = "root_pool/nix_build";
+      fsType = "zfs";
+    };
+
     "/var" = {
       device = "root_pool/var";
       fsType = "zfs";