<nix/buildenv.nix>: Don't substitute

This should get rid of a certificate warning from "nix-env -i" early
in the install script.

corepkgs/buildenv.nix

@@ -8,7 +8,7 @@ derivation {
   builder = perl;
   args = [ "-w" ./buildenv.pl ];
 
-  manifest = manifest;
+  inherit manifest;
 
   # !!! grmbl, need structured data for passing this in a clean way.
   derivations =
@@ -23,6 +23,9 @@ derivation {
   # network traffic, so don't do that.
   preferLocalBuild = true;
 
+  # Also don't bother substituting.
+  allowSubstitutes = false;
+
   __sandboxProfile = ''
     (allow sysctl-read)
     (allow file-read*