version: embed VERSION.txt in unstamped version

Temporary measure until we switch to Go 1.18.

    $ go run ./cmd/tailscale version
    1.17.0-date.20211022
      go version: go1.17

Updates #81

Change-Id: Ic82ebffa5f46789089e5fb9810b3f29e36a47f1a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

version-embed.go

@@ -0,0 +1,11 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package tailscaleroot embeds VERSION.txt into the binary.
+package tailscaleroot
+
+import _ "embed"
+
+//go:embed VERSION.txt
+var Version string

version/version.go

@@ -5,10 +5,20 @@
 // Package version provides the version that the binary was built at.
 package version
 
+import (
+	"strings"
+
+	tailscaleroot "tailscale.com"
+)
+
 // Long is a full version number for this build, of the form
 // "x.y.z-commithash", or "date.yyyymmdd" if no actual version was
 // provided.
-var Long = "date.20211004"
+var Long = "date.20211022"
+
+func init() {
+	Long = strings.TrimSpace(tailscaleroot.Version) + "-" + Long
+}
 
 // Short is a short version number for this build, of the form
 // "x.y.z", or "date.yyyymmdd" if no actual version was provided.