healthy: fix FILESEXTRAPATHS usage, HYP-10339

WARNING: FILESEXTRAPATHS-variable, must always use _prepend (or _append)
type of assignment, and don't forget the colon.
Please assign it with the format of:
  FILESEXTRAPATHS_append := ":${THISDIR}/Your_Files_Path" or
  FILESEXTRAPATHS_prepend := "${THISDIR}/Your_Files_Path:"
in your bbappend file

Signed-off-by: Steffen Sledz <sledz@dresearch-fe.de>