Fix TestBuildHermeticEnv cleanup race with Go telemetry background process

[Copilot]

TestBuildHermeticEnv fails intermittently on Go 1.26 Linux with TempDir RemoveAll cleanup: unlinkat .../telemetry: directory not empty. Go's telemetry subsystem spawns a background OS process that continues writing to $HOME/.config/go/telemetry/ after go mod init exits, racing against t.TempDir's RemoveAll cleanup.

Fix

Pre-create the telemetry mode file before running the hermetic build:

telemetryDir := filepath.Join(homeDir, ".config", "go", "telemetry")
os.MkdirAll(telemetryDir, 0o755)
os.WriteFile(filepath.Join(telemetryDir, "mode"), []byte("off\n"), 0o644)

Setting the mode to off prevents the background telemetry process from starting. GOTELEMETRY is not an overridable OS env var—it is read from this mode file—so this is the correct mechanism.

Fails only on Linux/Go 1.26 because macOS resolves the telemetry dir via ~/Library/... (outside the hermetic HOME), and the race window is narrower on earlier Go versions.