nixos/matrix-synapse: configure PYTHONMALLOC when using jemalloc

Per a synapse FAQ entry[1] this should be set in combination with\
jemalloc to make sure that jemalloc can accurately calculate memory
usage.

[1] https://github.com/element-hq/synapse/blob/develop/docs/usage/administration/admin_faq.md#help-synapse-is-slow-and-eats-all-my-ramcpu

nixos/modules/services/matrix/synapse.nix

@@ -1462,6 +1462,7 @@ in
         baseServiceConfig = {
           environment = optionalAttrs (cfg.withJemalloc) {
             LD_PRELOAD = "${pkgs.jemalloc}/lib/libjemalloc.so";
+            PYTHONMALLOC = "malloc";
           };
           serviceConfig = {
             Type = "notify";