Speed up evaluation of grids by evaluating custom packages once.

The current code evaluates our custom packages once for each node, which adds
signifcant amount of time to evaluate a grid. We can reduce this, by adding the
custom package set as an attribute to the nixpkgs set we pass to morph.

This doesn't change how we refer to those packages, as we continue to expose the
custom package set as a module attribute.

These are the times to evaluate all three grids (on a partially loaded system),
when there was nothing new to build:

```
before:
real	2m27.837s
user	3m35.528s
sys	0m3.722s

after:
real	1m12.748s
user	1m34.047s
sys	0m3.346s
```

morph/lib/default.nix

@@ -26,6 +26,13 @@
         "megacli"
       ];
     };
-    overlays = [];
+    # Expose `nixos/pkgs` as an attribute of our package set.
+    # This is is primarly consumed by `nixos/modules/packages.nix`, which
+    # then exposes it as a module argument. We do this here, so that
+    # the package set only needs to be evaluted once for the grid, rather
+    # than once for each host.
+    overlays = [
+      (self: super: { ourpkgs = self.callPackage ../../nixos/pkgs {}; })
+    ];
   };
 }

nixos/modules/packages.nix

 # A NixOS module which exposes custom packages to other modules.
 { pkgs, ...}:
 let
-  ourpkgs = pkgs.callPackage ../../nixos/pkgs {};
+  # Get our custom packages; either from the nixpkgs attribute added via an
+  # overlay in `morph/lib/default.nix`, or by importing them directly.
+  ourpkgs = pkgs.ourpkgs or (pkgs.callPackage ../pkgs {});
 in {
   config = {
     # Expose `nixos/pkgs` as a new module argument `ourpkgs`.

nixos/system-tests.nix

 # The overall system test suite for PrivateStorageio NixOS configuration.
 { pkgs }:
-{
-  private-storage = pkgs.nixosTest ./tests/private-storage.nix;
-  spending = pkgs.nixosTest ./tests/spending.nix;
-  tahoe = pkgs.nixosTest ./tests/tahoe.nix;
+let
+  # Add custom packages as an attribute, so it they only need to be evalutated once.
+  # See the comment in `morph/lib/default.nix` for details.
+  pkgs' = pkgs.extend (self: super: { ourpkgs = self.callPackage ./pkgs {}; });
+in {
+  private-storage = pkgs'.nixosTest ./tests/private-storage.nix;
+  spending = pkgs'.nixosTest ./tests/spending.nix;
+  tahoe = pkgs'.nixosTest ./tests/tahoe.nix;
 }