gccStdenv: actually force gcc if not already used

Previously the `cc` of the stdenv wasn't changed if `!stdenv.cc.isGNU`
which meant that gccStdenv didn't actually use gcc -- in contrast to all
the versioned `gcc*Stdenv`s.

pkgs/top-level/all-packages.nix

@@ -10402,7 +10402,11 @@ in
       };
     }) else ccWrapper;
 
-  gccStdenv = if stdenv.cc.isGNU then stdenv else stdenv.override {
+  gccStdenv =
+    if stdenv.cc.isGNU
+    then stdenv
+    else stdenv.override {
+      cc = buildPackages.gcc;
       allowedRequisites = null;
       # Remove libcxx/libcxxabi, and add clang for AS if on darwin (it uses
       # clang's internal assembler).