cmd/tailscale/cli: fix macOS crash reading envknob in init (#11667)

And add a test.

Regression from a5e1f7d703

Fixes tailscale/corp#19036

Change-Id: If90984049af0a4820c96e1f77ddf2fce8cb3043f

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

cmd/tailscale/cli/cli_test.go

@@ -16,6 +16,7 @@ import (
 
 	qt "github.com/frankban/quicktest"
 	"github.com/google/go-cmp/cmp"
+	"tailscale.com/envknob"
 	"tailscale.com/health/healthmsg"
 	"tailscale.com/ipn"
 	"tailscale.com/ipn/ipnstate"
@@ -33,6 +34,10 @@ import (
 // and at least one thing that's not (darwin, freebsd).
 var geese = []string{"linux", "darwin", "windows", "freebsd"}
 
+func TestPanicIfAnyEnvCheckedInInit(t *testing.T) {
+	envknob.PanicIfAnyEnvCheckedInInit()
+}
+
 // Test that checkForAccidentalSettingReverts's updateMaskedPrefsFromUpFlag can handle
 // all flags. This will panic if a new flag creeps in that's unhandled.
 //