toolchain.TestVersionSwitch = os.Getenv("TESTGO_VERSION_SWITCH")
		if v := os.Getenv("TESTGO_TOOLCHAIN_VERSION"); v != "" {
			work.ToolchainVersion = v
		}

		if testGOROOT := os.Getenv("TESTGO_GOROOT"); testGOROOT != "" {
			// Disallow installs to the GOROOT from which testgo was built.
			// Installs to other GOROOTs — such as one set explicitly within a test — are ok.
			work.AllowInstall = func(a *work.Action) error {