nixos/tests/acme: Patch certifi with cacert

Since 67651d80bc8baaf09ab91fec8ea423e09107ed8f the requests package now
depends on certifi, which in turn provides the CA root certificates that
we need to replace.

It might also be a good idea to actually patch certifi with our version
of cacert by default so that if we want to override and/or add something
we only need to do it once.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @fpletz, @k0ral, @FRidh

nixos/tests/acme.nix

@@ -14,10 +14,10 @@ let
 
       pythonPackages = (super.python.override {
         packageOverrides = lib.const (pysuper: {
-          requests = pysuper.requests.overrideDerivation (drv: {
+          certifi = pysuper.certifi.overrideDerivation (drv: {
             postPatch = (drv.postPatch or "") + ''
               cat "${self.cacert}/etc/ssl/certs/ca-bundle.crt" \
-                > requests/cacert.pem
+                > certifi/cacert.pem
             '';
           });
         });