clevis-luks-askpass: allow non-block-device volumes (#527)

Change the check on the volume that's being unlocked from
-b (checks for block device) to -e (checks if file exists).
This allows the code to work on LUKS volumes that are stored
in normal files.

Co-authored-by: x-qq <the@interwebs.test>

Author: x-qq

src/luks/systemd/clevis-luks-askpass.in

@@ -50,7 +50,7 @@ while true; do
             esac
         done < "$question"
 
-        [ -b "${d}" ] || continue
+        [ -e "${d}" ] || continue
         [ -S "${s}" ] || continue
 
         if ! pt="$(clevis_luks_unlock_device "${d}")" || [ -z "${pt}" ]; then