flake: use fast python for synapse in poseidon overlay

flake.nix

@@ -59,6 +59,17 @@
                 inherit system;
                 config.allowUnfree = true;
               };
+
+              fastPython3 = self.python3.override {
+                enableOptimizations = true;
+                reproducibleBuild = false;
+                self = self.fastPython3;
+                pythonAttr = "fastPython3";
+              };
+
+              matrix-synapse = super.matrix-synapse.override {
+                python3 = self.fastPython3;
+              };
             })
           ];
         }