ccache: fix references to stdenv.cc.cc.isClang

afa64a60 breaks the
tarball job https://hydra.nixos.org/build/40817224/nixlog/1/raw

The problem is that there is a stdenv.cc.cc.isGNU but no corresponding
stdenv.cc.cc.isClang. The "correct" fix would be to add
stdenv.cc.cc.isClang but this is simpler.