envknob: add GOOS func

Centralize the fake GOOS stuff, start to use it more. To be used more
in the future.

Change-Id: Iabacfbeaf5fca0b53bf4d5dbcdc0367f05a205f9
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

envknob/envknob_nottest.go

@@ -0,0 +1,17 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ts_not_in_tests
+
+package envknob
+
+import "runtime"
+
+func GOOS() string {
+	// When the "ts_not_in_tests" build tag is used, we define this func to just
+	// return a simple constant so callers optimize just as if the knob were not
+	// present. We can then build production/optimized builds with the
+	// "ts_not_in_tests" build tag.
+	return runtime.GOOS
+}