block-snapshot: fix ShellCheck warnings (2/3)
block-snapshot: fix ShellCheck warnings (2/3)

Forward-ported from qubes-core-agent-linux:

    commit aad6fa6d190d24393e326a4c2ff7ebc3b5921641
    Author: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
    Date:   Sat Sep 30 04:56:02 2017 +0200

        Hint shellcheck where to look for sourced files, if in repository

        This will ease running shellcheck from the repository.

marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin