[BUGFIX] Fix command detection with open_basedir in effect

When `open_basedir` is set (often the case on shared hosting), TYPO3
fails to find executables as it does a check for file existence first,
which fails due to the restriction.

Instead of checking file existence in all cases, a fallback has been
added on *nix systems that will try execution of a given command
and use the return code to decide whether the path is valid.

This itself uses the command `which` which is not available on Windows.
As nowadays `open_basedir` is mostly used on shared hosting
environments based on *nix, this will not be fixed for Windows.

Resolves: #70869
Resolves: #85174
Releases: main, 12.4
Change-Id: Ic01e822ae16b9ef1fa80be745a9de51e527ba059
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79865


Tested-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Benni Mack <benni@typo3.org>