all: use cmpx.Or where it made sense

I left a few out where writing it explicitly was better
for various reasons.

Updates #8296

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

cmd/get-authkey/main.go

@@ -16,6 +16,7 @@ import (
 
 	"golang.org/x/oauth2/clientcredentials"
 	"tailscale.com/client/tailscale"
+	"tailscale.com/util/cmpx"
 )
 
 func main() {
@@ -39,10 +40,7 @@ func main() {
 		log.Fatal("at least one tag must be specified")
 	}
 
-	baseURL := os.Getenv("TS_BASE_URL")
-	if baseURL == "" {
-		baseURL = "https://api.tailscale.com"
-	}
+	baseURL := cmpx.Or(os.Getenv("TS_BASE_URL"), "https://api.tailscale.com")
 
 	credentials := clientcredentials.Config{
 		ClientID:     clientID,