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.go

@@ -131,7 +131,7 @@ change in the future.
 			certCmd,
 			netlockCmd,
 			licensesCmd,
-			exitNodeCmd,
+			exitNodeCmd(),
 			updateCmd,
 			whoisCmd,
 		},

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.
 //

cmd/tailscale/cli/exitnode.go

@@ -20,7 +20,8 @@ import (
 	"tailscale.com/tailcfg"
 )
 
-var exitNodeCmd = &ffcli.Command{
+func exitNodeCmd() *ffcli.Command {
+	return &ffcli.Command{
 		Name:       "exit-node",
 		ShortUsage: "exit-node [flags]",
 		ShortHelp:  "Show machines on your tailnet configured as exit nodes",
@@ -59,6 +60,7 @@ var exitNodeCmd = &ffcli.Command{
 					},
 				}
 			})()...),
+	}
 }
 
 var exitNodeArgs struct {