"TESTGO_VCSTEST_TLS_HOST=" + httpsURL.Host,
		"TESTGO_VCSTEST_CERT=" + srvCertFile,
		"TESTGONETWORK=panic", // cleared by the [net] condition
		"GOSUMDB=" + testSumDBVerifierKey,
		"GONOPROXY=",
		"GONOSUMDB=",
		"GOVCS=*:all",
		"devnull=" + os.DevNull,
		"goversion=" + gover.Local(),
		"CMDGO_TEST_RUN_MAIN=true",
		"HGRCPATH=",
		"GOTOOLCHAIN=auto",
		"newline=\n",
	}

matching either pattern, including git.corp.example.com/xyzzy, rsc.io/private,
and rsc.io/private/quux.

For fine-grained control over module download and validation, the GONOPROXY
and GONOSUMDB environment variables accept the same kind of glob list
and override GOPRIVATE for the specific decision of whether to use the proxy
and checksum database, respectively.

	tmp := t.TempDir()
	if err := os.WriteFile(filepath.Join(tmp, "go.mod"), []byte("module m"), 0666); err != nil {
		t.Fatal(err)
	}
	t.Setenv("GO111MODULE", "on")
	t.Setenv("GOPROXY", "off")
	t.Setenv("GONOPROXY", "none")

	ctxt := Default
	ctxt.Dir = tmp

	pkgPath := "example.com/hello"
	_, err := ctxt.Import(pkgPath, tmp, FindOnly)
	if err == nil {
		t.Fatal("unexpected success")
	}

		Controls where various files are stored. See: 'go help gopath'.
	GOPROXY
		URL of Go module proxy. See https://golang.org/ref/mod#environment-variables
		and https://golang.org/ref/mod#module-proxy for details.
	GOPRIVATE, GONOPROXY, GONOSUMDB
		Comma-separated list of glob patterns (in the syntax of Go's path.Match)
		of module path prefixes that should always be fetched directly
		or that should not be compared against the checksum database.

//	GOPROXY
//		URL of Go module proxy. See https://golang.org/ref/mod#environment-variables
//		and https://golang.org/ref/mod#module-proxy for details.
//	GOPRIVATE, GONOPROXY, GONOSUMDB
//		Comma-separated list of glob patterns (in the syntax of Go's path.Match)
//		of module path prefixes that should always be fetched directly
//		or that should not be compared against the checksum database.