[TASK] Use environment variable for functional test bootstrap

Make it possible to determine the original root path
of the installation by setting an environment variable
just like in unit test bootstrap.

Releases: 6.2
Resolves: #56182
Change-Id: I42af5d4db20a23eb5d1ce90fc87b6df818a11e88
Reviewed-on: https://review.typo3.org/27771
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel