mpw: don't run checkPhase on aarch64

pkgs/tools/security/mpw/default.nix

···
     runHook postInstall
   '';
 
-  doCheck = true;
+  # Some tests are expected to fail on ARM64
+  # See: https://gitlab.com/spectre.app/cli/-/issues/27#note_962950844 (mpw is a predecessor to spectre-cli and this issue is relevant to mpw as well)
+  doCheck = !(stdenv.isLinux && stdenv.isAarch64);
 
   checkPhase = ''
     runHook preCheck