stdenv, cacert: consider $NIX_SSL_CERT_FILE in hooks

Some SSL libs don't react to $SSL_CERT_FILE.
That actually makes sense to me, as we add this behavior
as nixpkgs-specific, so it seems "safer" to use $NIX_*.

pkgs/data/misc/cacert/setup-hook.sh

@@ -1,4 +1,6 @@
 cacertHook() {
+    export NIX_SSL_CERT_FILE=@out@/etc/ssl/certs/ca-bundle.crt
+    # left for compatibility
     export SSL_CERT_FILE=@out@/etc/ssl/certs/ca-bundle.crt
 }