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

@@ -27,12 +27,10 @@ let ccache = stdenv.mkDerivation rec {
 
   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;