[postprocessor/ffmpeg] Fix finding ffprobe (bug in 21792b8) #2901

Job	Run time
Select tests from inputs	1s
Linter	16s
Run tests (ubuntu-20.04, 2.6, cpython, core, sh)	1m 21s
Run tests (ubuntu-20.04, 2.7, cpython, core, sh)	1m 6s
Run tests (ubuntu-20.04, 3.2, cpython, core, sh)	1m 6s
Run tests (ubuntu-20.04, 3.3, cpython, core, sh)	1m 12s
Run tests (ubuntu-20.04, 3.4, cpython, core, sh)	1m 22s
Run tests (ubuntu-20.04, 3.5, cpython, core, sh)	1m 10s
Run tests (ubuntu-20.04, 3.6, cpython, core, sh)	1m 16s
Run tests (ubuntu-20.04, 3.7, cpython, core, sh)	1m 7s
Run tests (ubuntu-20.04, 3.8, cpython, core, sh)	1m 13s
Run tests (ubuntu-20.04, 3.9, cpython, core, sh)	1m 10s
Run tests (ubuntu-20.04, 3.10, cpython, core, sh)	1m 4s
Run tests (ubuntu-20.04, 3.11, cpython, core, sh)	50s
Run tests (ubuntu-20.04, 3.12, cpython, core, sh)	1m 3s
Run tests (ubuntu-20.04, pypy-2.7, cpython, core, sh)	2m 19s
Run tests (ubuntu-20.04, pypy-3.6, cpython, core, sh)	2m 38s
Run tests (ubuntu-20.04, pypy-3.7, cpython, core, sh)	2m 33s
Run tests (windows-2019, 3.4, cpython, core, bat)	2m 7s
Run tests (windows-2019, 3.4, cpython, nodownload, bat)	48s
Run tests (ubuntu-20.04, 2.7, jython, core, sh)	3m 8s
Run tests (ubuntu-20.04, 2.7, jython, nodownload, sh)	22s
	29m 12s

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