sieve: offload mailserver.loginAccounts.<name>.sieveScript into /nix/store

This simplifies the remaining structure of `cfg.sieveDirectory`
a lot and gets us one step closer to removing
`activate-virtual-mail-users.service`.

mail-server/dovecot.nix

@@ -346,7 +346,19 @@ in
           "sieve_script default" = {
             # declarative
             type = "default";
-            path = "${cfg.sieveDirectory}/%{user}/default.sieve";
+            name = "default";
+            # TODO: Pre-compile Sieve scripts with 'sievec' (requires a Dovecot config in build sandbox)
+            path = "${
+              pkgs.runCommand "declarative-sieve-scripts" { } (
+                ''
+                  mkdir "$out"
+                ''
+                + lib.concatMapAttrsStringSep "\n" (_: value: ''
+                  mkdir "$out/${value.name}"
+                  cp -v "${builtins.toFile "default.sieve" value.sieveScript}" "$out/${value.name}/default.sieve"
+                '') (lib.filterAttrs (_: value: value.sieveScript != null) cfg.accounts)
+              )
+            }/%{user}/default.sieve";
           };
 
           "sieve_script personal" = {