borg-backup-server: set SHELL for verification script

In today's weird corner-case issue; when running under cron,
SHELL=/bin/sh ... which doesn't really matter (this script is run
under #!/bin/bash) *except* that "sudo -s" is obeying SHELL and
consequently the in-line script here fails under cron, but not when
run interactively.  Just set SHELL=/bin/bash for consistency.

Change-Id: Ic8584b90fea8382f7a7d294b98a0a3689bfc981b

playbooks/roles/borg-backup-server/files/verify-borg-backups.sh

@@ -1,5 +1,9 @@
 #!/bin/bash
 
+# When under cron, SHELL can bin /bin/sh which the "sudo -s" below
+# obeys.  Let's just use bash.
+export SHELL=/bin/bash
+
 pushd /opt/backups
 
 for u in borg-*; do