Use internal BASH_VERSINFO variable

Signed-off-by: Knut Ahlers <knut@ahlers.me>

bin/ensure-config.sh

@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
-( bash --version | grep -q 'version 4' ) || {
+[ ${BASH_VERSINFO[0]:-0} -lt 4 ] && {
   echo "Bash too old, update to >=4.0"
   exit 1
 }