renge/mautrix-signal: Allow file transfer

mautrix-whatsapp places a file in /tmp/ for file transfers. However,
signald is running with PrivateTemp=true. This uses systemd’s feature
that allows a unit to share a namespace with another one and places
mautrix-signal in the same namespace as signald, so they share their
private tmp directory.

machines/renge/services/matrix/mautrix-signal.nix

@@ -85,12 +85,17 @@ in
 
     serviceConfig = {
       DynamicUser = true;
+      PrivateTmp = true;
       SupplementaryGroups = [ "signald" ];
       StateDirectory = "mautrix-signal";
       WorkingDirectory = "/var/lib/mautrix-signal";
       ExecStart = "${pkgs.mautrix-signal}/bin/mautrix-signal -c ${generatedConfig}/config.yaml";
       Restart = "on-failure";
     };
+
+    unitConfig = {
+      JoinsNamespaceOf = "signald.service";
+    };
   };
 
   services.matrix-synapse.settings.app_service_config_files = lib.singleton "${generatedConfig}/registration.yaml";