Get rid of all with { inherit... } and just used let inherit...

The old forms presumably predates, or were made in ignorance of,
`let inherit`. This way is better style as the scoping as more lexical,
something which Nix can (or might already!) take advantage of.

pkgs/servers/coturn/default.nix

@@ -1,6 +1,6 @@
 { stdenv, fetchFromGitHub, openssl, libevent }:
 
-with { inherit (stdenv.lib) optional; };
+let inherit (stdenv.lib) optional; in
 
 stdenv.mkDerivation rec {
   name = "coturn-${version}";