ci: store Clamav DB in LFS

Fix #136 (stop pulling the files from @griff's poor server), also add a script
to update the files.

The fun thing about this is that due to sourcing the files from
`https://gitlab.com/simple-nixos-mailserver/nixos-mailserver/raw/master/tests/clamav`
during the tests, updates to the `hashes.json` and `*.cvd` files will always
fail CI.  I guess this is a reasonable tradeoff as long as people are aware of
it.

shell.nix

@@ -0,0 +1,9 @@
+{ nixpkgs ? <nixpkgs>, system ? builtins.currentSystem }:
+
+with (import nixpkgs { inherit system; }); stdenv.mkDerivation rec {
+  name = "nixos-mailserver-env";
+  env = buildEnv { name = name; paths = buildInputs; };
+  buildInputs = with pkgs; [
+    jq clamav
+  ];
+}