stdenvAdapters: fix preservation of env.NIX_CFLAGS_COMPILE

we have managed to migrate to NIX_CFLAGS_COMPILE to the env attrset well
enough that we don't need to support having it toplevel. mkDerivation
will throw if there's a attr in both env and toplevel so no need to
worry about that

pkgs/stdenv/adapters.nix

···
     stdenv.override (old: {
       mkDerivationFromStdenv = extendMkDerivationArgs old (args: {
         dontStrip = true;
-        env = (args.env or {}) // { NIX_CFLAGS_COMPILE = toString (args.NIX_CFLAGS_COMPILE or "") + " -ggdb -Og"; };
+        env = (args.env or {}) // { NIX_CFLAGS_COMPILE = toString (args.env.NIX_CFLAGS_COMPILE or "") + " -ggdb -Og"; };
       });
     });
 
···
   impureUseNativeOptimizations = stdenv:
     stdenv.override (old: {
       mkDerivationFromStdenv = extendMkDerivationArgs old (args: {
-        env = (args.env or {}) // { NIX_CFLAGS_COMPILE = toString (args.NIX_CFLAGS_COMPILE or "") + " -march=native"; };
+        env = (args.env or {}) // { NIX_CFLAGS_COMPILE = toString (args.env.NIX_CFLAGS_COMPILE or "") + " -march=native"; };
 
         NIX_ENFORCE_NO_NATIVE = false;
 
···
   withCFlags = compilerFlags: stdenv:
     stdenv.override (old: {
       mkDerivationFromStdenv = extendMkDerivationArgs old (args: {
-        env = (args.env or {}) // { NIX_CFLAGS_COMPILE = toString (args.NIX_CFLAGS_COMPILE or "") + " ${toString compilerFlags}"; };
+        env = (args.env or {}) // { NIX_CFLAGS_COMPILE = toString (args.env.NIX_CFLAGS_COMPILE or "") + " ${toString compilerFlags}"; };
       });
     });
 }