[BUGFIX] Include keys in YamlFileLoader placeholder processing

This commit allows to use environment variables in YAML keys as well.

Resolves: #99889
Releases: main, 12.4
Change-Id: I1e0272aade1188a483089412156c18e291d1ffbc
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84891


Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>