treefmt: collapse the assertion-message interpolation to one line

nixfmt wanted the ${...} on a single line (verified locally:
nix build .#checks.x86_64-linux.formatting passes). The earlier CI 'docs'
failure was a transient muede-pc2 build hiccup — the docs check builds clean
locally on the same drv.

nix/modules/hive-forge.nix

@@ -355,9 +355,7 @@ in
         assertion = lib.all (m: lib.length (lib.splitString "/" m.dest) == 2) effectiveMirrors;
         message = ''
           Every services.hyperhive.forge.mirrors[].dest must be exactly
-          "<owner>/<repo>" (one slash). Got: ${
-            lib.concatMapStringsSep ", " (m: m.dest) effectiveMirrors
-          }
+          "<owner>/<repo>" (one slash). Got: ${lib.concatMapStringsSep ", " (m: m.dest) effectiveMirrors}
         '';
       }
       {