buildenv: Make buildEnv overridable

callPackage already calls makeOverridable, but that just
makes the function that evaluates to buildEnv overridable,
not buildEnv itself.
If no overridable version of buildEnv is used during construction,
users can't override e.g. `paths` at all

pkgs/build-support/buildenv/default.nix

@@ -4,7 +4,8 @@
 
 { buildPackages, runCommand, lib }:
 
-{ name
+lib.makeOverridable
+({ name
 
 , # The manifest file (if any).  A symlink $out/manifest will be
   # created to it.
@@ -68,4 +69,4 @@ runCommand name
   ''
     ${buildPackages.perl}/bin/perl -w ${./builder.pl}
     eval "$postBuild"
-  ''
+  '')