stalwart-mail: add rocksdb to passthrough

Stalwart Mail needs a specific version of rocksdb (in particular,
stalwart-mail 0.10.5 does not support rocksdb 9).
This makes creating backup scripts harder, since the version of
rocksdb.tools should match the version in use by stalwart-mail.
At the moment, there doesn't seem to be a way to get the version
of rocksdb used to build stalwart-mail from its derivation, so this
commits adds it to passthrough.

pkgs/by-name/st/stalwart-mail/package.nix

@@ -149,6 +149,7 @@ rustPlatform.buildRustPackage {
   doCheck = !(stdenv.hostPlatform.isLinux && stdenv.hostPlatform.isAarch64);
 
   passthru = {
+    inherit rocksdb; # make used rocksdb version available (e.g., for backup scripts)
     webadmin = callPackage ./webadmin.nix { };
     updateScript = nix-update-script { };
     tests.stalwart-mail = nixosTests.stalwart-mail;