[external/FFmpeg] Fix and improve --ffmpeg-location handling #863

Job	Run time
Linter	18s
Select tests from inputs	1s
Run tests (ubuntu-20.04, 2.6, cpython, core, sh)	1m 15s
Run tests (ubuntu-20.04, 2.7, cpython, core, sh)	1m 0s
Run tests (ubuntu-20.04, 3.2, cpython, core, sh)	57s
Run tests (ubuntu-20.04, 3.3, cpython, core, sh)	1m 17s
Run tests (ubuntu-20.04, 3.4, cpython, core, sh)	1m 12s
Run tests (ubuntu-20.04, 3.5, cpython, core, sh)	1m 12s
Run tests (ubuntu-20.04, 3.6, cpython, core, sh)	1m 12s
Run tests (ubuntu-20.04, 3.7, cpython, core, sh)	1m 13s
Run tests (ubuntu-20.04, 3.8, cpython, core, sh)	1m 8s
Run tests (ubuntu-20.04, 3.9, cpython, core, sh)	1m 6s
Run tests (ubuntu-20.04, 3.10, cpython, core, sh)	1m 12s
Run tests (ubuntu-20.04, 3.11, cpython, core, sh)	1m 14s
Run tests (ubuntu-20.04, 3.12, cpython, core, sh)	1m 13s
Run tests (ubuntu-20.04, pypy-2.7, cpython, core, sh)	2m 25s
Run tests (ubuntu-20.04, pypy-3.6, cpython, core, sh)	2m 35s
Run tests (ubuntu-20.04, pypy-3.7, cpython, core, sh)	2m 34s
Run tests (windows-2019, 3.4, cpython, core, bat)	1m 57s
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 19s

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