flake: migrate from cachix to cache.numtide.com (#655)

Numtide is migrating from Cachix to a self-hosted binary cache at
cache.numtide.com. Update the substituter URL and public key to use
the new infrastructure.

Author: Mic92

flake.nix

@@ -2,8 +2,8 @@
   description = "treefmt: the formatter multiplexer";
 
   nixConfig = {
-    extra-substituters = ["https://numtide.cachix.org"];
-    extra-trusted-public-keys = ["numtide.cachix.org-1:2ps1kLBUWjxIneOy1Ik6cQjb41X0iXVXeHigGmycPPE="];
+    extra-substituters = ["https://cache.numtide.com"];
+    extra-trusted-public-keys = ["niks3.numtide.com-1:DTx8wZduET09hRmMtKdQDxNNthLQETkc/yaX7M4qK0g="];
   };
 
   inputs = {