version: validate Long format on Android builds

Updates #14069

Change-Id: I134a90db561dacc4b1c1c66ccadac135b5d64cf3
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

version/version_checkformat.go

@@ -0,0 +1,17 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build tailscale_go && android
+
+package version
+
+import "fmt"
+
+func init() {
+	// For official Android builds using the tailscale_go toolchain,
+	// panic if the builder is screwed up we fail to stamp a valid
+	// version string.
+	if !isValidLongWithTwoRepos(Long()) {
+		panic(fmt.Sprintf("malformed version.Long value %q", Long()))
+	}
+}