fail2ban: fix dovecot filter's journalmatch

The dovecot filter config distributed with fail2ban watches for log
entries with a journalmatch filter of "_SYSTEMD_UNIT=dovecot.service",
but NixOS's Dovecot runs as dovecot2.service. Fix the journalmatch
config to match NixOS's Dovecot.

pkgs/tools/security/fail2ban/default.nix

@@ -21,6 +21,9 @@ pythonPackages.buildPythonApplication {
         --replace /usr/sbin/sendmail sendmail \
         --replace /usr/bin/whois whois
     done
+
+    substituteInPlace config/filter.d/dovecot.conf \
+      --replace dovecot.service dovecot2.service
   '';
 
   doCheck = false;