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

afa64a60bc73a5b5b1340f9ec9daae198d49e5d2 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.

SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs