matrix-synapse: disable test parallelism on aarch64-linux

No idea why, but the tests - a random amount of tests to be precise -
fail on aarch64 on each attempt I made. Not reproducible on
x86_64-linux. Disabling parallelism appears to solve the issue.

pkgs/servers/matrix-synapse/default.nix

@@ -86,13 +86,13 @@ buildPythonApplication rec {
 
   doCheck = !stdenv.isDarwin;
 
-  checkPhase = ''
+  checkPhase = let testFlags = lib.optionalString (!stdenv.isAarch64) "-j $NIX_BUILD_CORES"; in ''
     runHook preCheck
 
     # remove src module, so tests use the installed module instead
     rm -rf ./synapse
 
-    PYTHONPATH=".:$PYTHONPATH" ${python3.interpreter} -m twisted.trial -j $NIX_BUILD_CORES tests
+    PYTHONPATH=".:$PYTHONPATH" ${python3.interpreter} -m twisted.trial ${testFlags} tests
 
     runHook postCheck
   '';