asusmini: resolve agenix rules explicitly during cutover

scripts/upgrade-knot.sh

@@ -151,7 +151,8 @@ trap 'exit 143' TERM
 
 umask 077
 runtime=$(mktemp -d "$runtime_template")
-agenix -d "$repo/secrets/knot-env.age" -i "$ssh_key" > "$runtime/knot.env"
+AGENIX_RULES="$repo/secrets/secrets.nix" \
+  agenix -d knot-env.age -i "$ssh_key" > "$runtime/knot.env"
 chmod 0400 "$runtime/knot.env"
 master_key=""
 export -n master_key