Fix some small bugs in install.sh

The `install.sh` script contains "bashisms" that don't work with POSIX `sh`, or
`bash` in `sh` mode. The location of `bash` on FreeBSD is different than most
Linux distributions so the use of `/usr/bin/env` to find the installed bash is a
safe cross-platform way of launching this script as described in the `README`.

Modified the pre-installation check to test if the currently installed version
is the same rather than just for the presence of the `atomvm_packbeam`
directory so that upgrading to other versions will be possible with future
releases.

Signed-off-by: Winford <[email protected]>

install.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/usr/bin/env bash
 #
 # This file is part of AtomVM.
 #
@@ -84,8 +84,8 @@ echo_run cp "${src_tar}" "${tmp_dir}/."
 echo_run gunzip "${tmp_dir}/atomvm_packbeam-${version}.tar.gz"
 
 readonly dest_dir="${prefix}/atomvm_packbeam"
-if [ -e "${dest_dir}" ]; then
-    echo "ERROR! It looks like ${dest_dir} already exists!"
+if [ $(${dest_dir}/bin/packbeam version) = ${version} ]; then
+    echo "ERROR! It looks like ${version} is already installed!"
     exit 1
 fi