ccache.links: make it a function of unwrappedCC

That way we can use a ccacheStdenv with e.g. a different CC:

stdenv = overrideCC gcc8Stdenv (ccacheWrapper.override { unwrappedCC
= gcc8Stdenv.cc.cc; });

pkgs/development/tools/misc/ccache/default.nix

···
 
   doCheck = !stdenv.isDarwin;
 
-  passthru = let
-      unwrappedCC = stdenv.cc.cc;
-    in {
+  passthru = {
     # A derivation that provides gcc and g++ commands, but that
     # will end up calling ccache for the given cacheDir
-    links = extraConfig: stdenv.mkDerivation rec {
+    links = {unwrappedCC, extraConfig}: stdenv.mkDerivation rec {
       name = "ccache-links";
       passthru = {
         isClang = unwrappedCC.isClang or false;

pkgs/top-level/all-packages.nix

···
   #     };
   # You can use a different directory, but whichever directory you choose
   # should be owned by user root, group nixbld with permissions 0770.
-  ccacheWrapper = makeOverridable ({ extraConfig ? "" }:
-     wrapCC (ccache.links extraConfig)) {};
+  ccacheWrapper = makeOverridable ({ extraConfig ? "", unwrappedCC ? stdenv.cc.cc }:
+     wrapCC (ccache.links {inherit unwrappedCC extraConfig;})) {};
   ccacheStdenv = lowPrio (overrideCC stdenv buildPackages.ccacheWrapper);
 
   cccc = callPackage ../development/tools/analysis/cccc { };