[BUGFIX] Add missing composer.json adjustments for ext-libxml

With #91069, the check for the System Environment as well as the
definition of the newly introduced requirement of ext-libxml went lost
during review. The patch adds them again, as is needed.

Resolves: #91730
Releases: master, 10.4
Change-Id: I3bf8fd3aaeb52d7bceb588f200c6dc29eb1ad58e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64953


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>