fixup: Update README to mention the need to vendor tmpnet

Author: maru-ava

tests/fixture/tmpnet/README.md

@@ -411,9 +411,9 @@ Example usage:
     loki_password: ${{ secrets.LOKI_PASSWORD || '' }}
 ```
 
-The action assumes a nix flake file in the repo root that enables
+The action requires a flake.nix file in the repo root that enables
 availability of promtail and prometheus. The following is a minimal
-flake file that inherits from the avalanchego flake:
+flake that inherits from the avalanchego flake:
 
 ```nix
 {
@@ -442,6 +442,18 @@ flake file that inherits from the avalanchego flake:
 }
 ```
 
+The action also requires being able to invoke tmpnetctl via `go
+run`. Use of a `tools.go` file that imports tmpnetctl is suggested to
+enable this:
+
+```golang
+package tools
+
+import (
+	_ "github.com/ava-labs/avalanchego/tests/fixture/tmpnet/cmd" // tmpnetctl
+)
+```
+
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