Improve remote store eval time

This code used to put the .drv path into the string context for each
package. That meant we had to query the store each time one of these was
evaluated for the out path. This is pretty quick when using a non-remote
store. But the latency with the remote store really slows us down.

We should just calculate the outPath once instead, and use that
throughout.

Author: TeofilC

lib/load-cabal-plan.nix

@@ -75,7 +75,9 @@ let
           buildable = true;
         } // lookupDependencies hsPkgs.pkgsBuildBuild (components.setup.depends or []) (components.setup.exe-depends or []);
       };
-  nixFilesDir = callProjectResults.projectNix + callProjectResults.src.origSubDir or "";
+  # We use unsafeDiscardStringContext to ensure that we don't query this derivation when importing
+  # each package. The cost can be very high when using a remote store, as we need to do a network call.
+  nixFilesDir = builtins.unsafeDiscardStringContext callProjectResults.projectNix.outPath + callProjectResults.src.origSubDir or "";
 in {
   # This replaces the `plan-nix/default.nix`
   pkgs = (hackage: {