Enable Nix and refresh linux cache volumes

services/forgejo-nsc/README.md

@@ -48,7 +48,8 @@ profile. The important knobs are:
 - `namespace.linux_cache_*` / `namespace.macos_cache_*` – persistent cache
   volumes mounted into runners so Linux can keep `/nix` plus shared build
   caches warm and macOS can reuse Rust toolchains, Xcode package caches, and
-  lane-local derived data.
+  lane-local derived data. If Namespace keeps reusing an older undersized cache
+  volume, bump the cache tag name to force a fresh allocation at the new size.
 
 ### Running locally