self-test: use correct executable
requested to merge work/heirecka/cherry-pick-7173c1e514ee3d4f89b5bce799093779f3c1d870 into release/24.12
Use the built target file instead of the 'telly-skout' executable on PATH, which most likely isn't the one just built and, in the case of an upgrade from 24.08.3 to >= 24.11.80 doesn't have the --self-test option, resulting in a failed test.
(cherry picked from commit 7173c1e5)