packages: convert path interpolation to concat

I have no idea what the fuck is happening here but interpolating the
path leads to the whole result being converted to a (-n invalid) store
path, but concat doesn't. The former results in realisation failing
running under `nix-build`, i.e., NUR updates.

callUnitRoot.nix

@@ -7,9 +7,9 @@
 let
   shards = lib.attrNames (builtins.readDir root);
   namesForShard = shard: lib.mapAttrs'
-    (name: _: { inherit name; value = "${root}/${shard}/${name}"; })
-    (builtins.readDir "${root}/${shard}");
+    (name: _: { inherit name; value = root + "/${shard}/${name}"; })
+    (builtins.readDir (root + "/${shard}"));
   namesToPath = lib.foldl' lib.recursiveUpdate { } (map namesForShard shards);
-  units = lib.mapAttrs (_: path: pkgs.callPackage "${path}/${packageFun}" { }) namesToPath;
+  units = lib.mapAttrs (_: path: pkgs.callPackage (path + "/${packageFun}") { }) namesToPath;
 in
 units