Disable building in chroot for Nix's corepkgs

The dependencies of the corepkgs are not necessarily in the chroot (or
in the Nix store), so don't build them in a chroot.

corepkgs/buildenv.nix

@@ -18,4 +18,7 @@ derivation {
   # Building user environments remotely just causes huge amounts of
   # network traffic, so don't do that.
   preferLocalBuild = true;
+
+  # Don't build in a chroot because Nix's dependencies may not be there.
+  __noChroot = true;
 }

corepkgs/nar.nix

@@ -28,4 +28,7 @@ derivation {
   builder = shell;
   args = [ "-e" builder ];
   inherit storePath hashAlgo;
+
+  # Don't build in a chroot because Nix's dependencies may not be there.
+  __noChroot = true;
 }

corepkgs/unpack-channel.nix

@@ -8,6 +8,10 @@ derivation {
   args = [ "-e" ./unpack-channel.sh ];
   inherit name channelName src bzip2 tar tr;
   PATH = "${nixBinDir}:${coreutils}";
+  
   # No point in doing this remotely.
   preferLocalBuild = true;
+
+  # Don't build in a chroot because Nix's dependencies may not be there.
+  __noChroot = true;
 }