Use findBinaryPath for previews

[J0WI]

findBinaryPath uses cache:

server/lib/private/legacy/OC_Helper.php

Lines 460 to 464 in 4be2e16

	public static function findBinaryPath($program) {
	$memcache = \OC::$server->getMemCacheFactory()->createDistributed('findBinaryPath');
	if ($memcache->hasKey($program)) {
	return $memcache->get($program);
	}

I think avconv can be removed completely in near future, the fork is no longer maintained: https://lists.libav.org/pipermail/libav-devel/2020-April/086589.html

[kesselb]

Fine by me to remove avconv right away.

[skjnldsv]

Please rebase :)