Move some corepkgs into the nix binary

src/nix-env/buildenv.nix

@@ -0,0 +1,25 @@
+{ derivations, manifest }:
+
+derivation {
+  name = "user-environment";
+  system = "builtin";
+  builder = "builtin:buildenv";
+
+  inherit manifest;
+
+  # !!! grmbl, need structured data for passing this in a clean way.
+  derivations =
+    map (d:
+      [ (d.meta.active or "true")
+        (d.meta.priority or 5)
+        (builtins.length d.outputs)
+      ] ++ map (output: builtins.getAttr output d) d.outputs)
+      derivations;
+
+  # Building user environments remotely just causes huge amounts of
+  # network traffic, so don't do that.
+  preferLocalBuild = true;
+
+  # Also don't bother substituting.
+  allowSubstitutes = false;
+}