nixos/matrix-authentication-service: use env var instead of %d

%d works in the context of the ExecStart command, since it's expanded by
systemd. However, extraConfigFiles is also used in the ExecStartPre
*script*, and bash does not know what %d is, so it's not expanded and
the MAS config check fails because it can't find "%d/config-0" (instead
of the intended "/run/credentials/...")

nixos/modules/services/matrix/matrix-authentication-service.nix

@@ -45,7 +45,9 @@ let
       pruned;
   configFile = format.generate "config.yaml" finalSettings;
 
-  extraConfigFiles = lib.imap0 (i: _: "%d/config-${toString i}") cfg.extraConfigFiles;
+  extraConfigFiles = lib.imap0 (
+    i: _: "\${CREDENTIALS_DIRECTORY}/config-${toString i}"
+  ) cfg.extraConfigFiles;
   runtimeConfig = "/run/matrix-authentication-service/config.yaml";
 in
 {