cmd/printdep: add flag to print out Go toolchain tarball URL

Updates #3669

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

cmd/printdep/printdep.go

@@ -9,6 +9,8 @@ package main
 import (
 	"flag"
 	"fmt"
+	"log"
+	"runtime"
 	"strings"
 
 	ts "tailscale.com"
@@ -16,6 +18,7 @@ import (
 
 var (
 	goToolchain    = flag.Bool("go", false, "print the supported Go toolchain git hash (a github.com/tailscale/go commit)")
+	goToolchainURL = flag.Bool("go-url", false, "print the URL to the tarball of the Tailscale Go toolchain")
 )
 
 func main() {
@@ -23,4 +26,21 @@ func main() {
 	if *goToolchain {
 		fmt.Println(strings.TrimSpace(ts.GoToolchainRev))
 	}
+	if *goToolchainURL {
+		var suffix string
+		switch runtime.GOARCH {
+		case "amd64":
+			// None
+		case "arm64":
+			suffix = "-" + runtime.GOARCH
+		default:
+			log.Fatalf("unsupported GOARCH %q", runtime.GOARCH)
+		}
+		switch runtime.GOOS {
+		case "linux", "darwin":
+		default:
+			log.Fatalf("unsupported GOOS %q", runtime.GOOS)
+		}
+		fmt.Printf("https://github.com/tailscale/go/releases/download/build-%s/%s%s.tar.gz\n", strings.TrimSpace(ts.GoToolchainRev), runtime.GOOS, suffix)
+	}
 }