tailscale-version.sh: always use bash

We specify `set -o pipefail`, which is not implemented in dash
or a number of other shells. The F-Droid builder is defaulting
to a shell which does not:
```
./version/tailscale-version.sh: 10: set: Illegal option -o pipefail
```

Updates https://github.com/tailscale/tailscale/issues/2536

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