nix: fallback enabled

Dumb to need a config to say this...

Author: khaneliman

modules/shared/nix/default.nix

@@ -248,6 +248,8 @@ in
             "pipe-operators"
             "dynamic-derivations"
           ];
+          # Prevent builds failing just because we can't contact a substituter
+          fallback = true;
           flake-registry = "/etc/nix/registry.json";
           http-connections = 0;
           keep-derivations = true;