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.

tests/clamav/.gitattributes

@@ -0,0 +1 @@
+*cvd filter=lfs diff=lfs merge=lfs -text

tests/clamav/.gitignore

@@ -0,0 +1 @@
+mirrors.dat

tests/clamav/freshclam.conf

@@ -0,0 +1 @@
+DatabaseMirror database.clamav.net

tests/clamav/hashes.json

@@ -0,0 +1,5 @@
+{
+  "bytecode.cvd": "a6800da4e3740b611e4f8a8e835be4a867abf8009af502e5bbf038d3ad162fa8",
+  "daily.cvd": "da96006e191412806bac1a2cef5d48ed0ec1b46efa150cf0aa6c27e734f4c4f8",
+  "main.cvd": "081884225087021e718599e8458ff6c9ee3cdebed8775dd8e445fc7b589d88a6"
+}

tests/clamav/update-clamav-database.sh

@@ -0,0 +1,15 @@
+#!/bin/sh
+
+set -e
+
+cd "$(dirname "${0}")"
+
+rm ./*.cvd hashes.json || :
+
+freshclam --datadir=. --config-file=freshclam.conf
+(for i in ./*.cvd;
+ do echo '{}' |
+     jq --arg path "$(basename "${i}")" \
+        --arg sha256sum "$(sha256sum "${i}" | awk '{ print $1; }')" \
+        '.[$path] = $sha256sum'; done) |
+  jq -s add > hashes.json