version: use build_dist.sh shellvars

The Tailscale OSS repo has removed version/version.sh, use
`build_dist.sh shellvars` instead.

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

version/tailscale-version.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/env sh
+#!/usr/bin/env bash
 
 # Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
 # Use of this source code is governed by a BSD-style
@@ -37,5 +37,5 @@ git reset --hard -q
 git clean -d -x -f
 git fetch -q --all --tags
 git checkout -q "$mod_version"
-eval $( ./version/version.sh)
+eval $( ./build_dist.sh shellvars)
 echo $VERSION_LONG