nixos/tests/acme: Patch certifi with cacert

Since 67651d80

 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