go.toolchain.rev, version: bump Tailscale Go, add IsTailscaleGo

Reports whether the current binary was built with Tailscale's
custom Go toolchain (the "tailscale_go" build tag).

For https://github.com/tailscale/go/pull/165

Updates tailscale/corp#39430

Change-Id: Ica437582ddf55d7df48b1453bad03ce14b1c0949
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

version/version.go

@@ -172,6 +172,10 @@ func majorMinorPatch() string {
 	return ret
 }
 
+// IsTailscaleGo reports whether the current binary was built with
+// Tailscale's custom Go toolchain.
+func IsTailscaleGo() bool { return isTailscaleGo }
+
 func isValidLongWithTwoRepos(v string) bool {
 	s := strings.Split(v, "-")
 	if len(s) != 3 {

version/version_not_tsgo.go

@@ -0,0 +1,8 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !tailscale_go
+
+package version
+
+const isTailscaleGo = false

version/version_tsgo.go

@@ -0,0 +1,8 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build tailscale_go
+
+package version
+
+const isTailscaleGo = true