adding root_pool/nix_build to all zfs boxes

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

systems/rhapsody-in-green/hardware.nix

@@ -34,14 +34,22 @@
       device = "root_pool/root";
       fsType = "zfs";
     };
+
     "/home" = {
       device = "root_pool/home";
       fsType = "zfs";
     };
+
+    "/nix_build" = {
+      device = "root_pool/nix_build";
+      fsType = "zfs";
+    };
+
     "/var" = {
       device = "root_pool/var";
       fsType = "zfs";
     };
+
     "/boot" = {
       device = "/dev/disk/by-uuid/12CE-A600";
       fsType = "vfat";