Add Nix flake for mcproxyctl

Vendor dependencies and expose mcproxyctl binary via nix build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/github.com/oschwald/maxminddb-golang/set_zero_120.go

@@ -0,0 +1,10 @@
+//go:build go1.20
+// +build go1.20
+
+package maxminddb
+
+import "reflect"
+
+func reflectSetZero(v reflect.Value) {
+	v.SetZero()
+}