ccache: Add meta.mainProgram to links

Inherit meta.mainProgram from unwrappedCC, which should have
meta.mainProgram set.

Fixes #437074

pkgs/by-name/cc/ccache/package.nix

···
               ln -s ${unwrappedCC}/$file $out/$file
             done
           '';
+
+        meta = {
+          inherit (unwrappedCC.meta) mainProgram;
+        };
       };
 
     updateScript = nix-update-script { };