Specify shell for bin/get_eigen.sh script

This commit prevents errors for people using sh, zsh, or other
shells that don't support '[[' syntax. All the other scripts of the
form bing/get_*.sh specify the shell.

bin/get_eigen.sh

@@ -1,3 +1,4 @@
+#! /bin/bash -e
 # SPDX-License-Identifier: EPL-2.0 OR GPL-2.0-or-later
 # SPDX-FileCopyrightText: Bradley M. Bell <[email protected]>
 # SPDX-FileContributor: 2003-24 Bradley M. Bell