stdenv: label the ephemeral coreutils-stage4 package

During stdenv bootstrapping, coreutils is built twice.  This makes
troubleshooting very difficult, because both packages have
name="coreutils", so it is a hassle to figure out "which coreutils am
I using / is not building"?

The first of these builds is used only in stage4, and is not part of
the final stdenv.  Let's label that one with a different `name`
attribute to make it obvious which is which.

pkgs/stdenv/linux/default.nix

@@ -351,6 +351,9 @@ in
       #   stage5.gcc -> stage4.coreutils -> stage3.glibc -> bootstrap
       gmp = super.gmp.override { stdenv = self.stdenv; };
 
+      # coreutils gets rebuilt both here and also in the final stage; we rename this one to avoid confusion
+      coreutils = super.coreutils.overrideAttrs (a: a // { name = "coreutils-stage4"; });
+
       gcc = lib.makeOverridable (import ../../build-support/cc-wrapper) {
         nativeTools = false;
         nativeLibc = false;