fix tests on nixos-unstable

symphorienrequested to merge
symphorien/nixos-mailserver:nixos-unstable-compat into master

Also update nix flake inputs now that tests pass

About header changes: since nixos tests also have ipv6, rspamd has increased its spam score because of HFILTER_HELO_IP_A (is the ipv6 reverse dns broken ?) in the multiple.nix test. This made email sent by the test go above a threshold and broke the test. I tried to fix the dns issue but failed. Instead, I decreased the spam score by fixing pre-existing but unrelated issues: missing Message-Id and Date headers.

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