PHP discovery on Windows uses current drive instead of system drive

[supervirus]

Currently the function systemDir in discovery_windows.go return the current drive instead of the system drive.
For the majority of the developers this is no problem, as they only use a single drive for both Windows and projects.
In the future this can change, as Microsoft pushes developers to use DevDrives.

The system drive is available in the environment variable SystemDrive.
Using this environment variable instead of the current drive would reflect the behaviour of the Composer Windows installer, which the discovery is based on.

Affected version:
Symfony CLI version 5.8.11

[tucksaun]

makes sense. willing to open a PR?