make cache dir configurable

impl.nix

@@ -109,16 +109,12 @@ EOF
       '';
 
   denoWithCache =
-    { baseDeno, sharedCache }:
+    { baseDeno, sharedCache, dir }:
     pkgs.writeShellScriptBin "deno" ''
       set -euo pipefail
 
-      if [ -n "''${FLAKE_ROOT:-}" ]; then
-        CACHE_DIR="$FLAKE_ROOT/.deno_cache"
-      else
-        echo "FLAKE_ROOT not set. Make sure you're using the flake-root devShell." >&2
-        exit 1
-      fi
+      # Use configured dir, with support for FLAKE_ROOT variable expansion
+      CACHE_DIR="${dir}"
 
       if [ ! -d "$CACHE_DIR" ] || [ ! "$(ls -A "$CACHE_DIR" 2>/dev/null)" ]; then
         echo "Setting up Deno cache at: $CACHE_DIR" >&2