Why can't I install Git for Windows with OpenSSH?

Why can't I install Git for Windows with OpenSSH?

Whenever I try to install Git for Windows, using the installer downloaded
from here, the installation seemingly skips the step where I configure my
Secure Shell client for Git.
Normally during installation, a screen prompting the user to choose one of
either OpenSSH (installed with Git) or PuTTY PLink (the default option) is
shown. With my latest few installs, this screen is nowhere to be found.
Instead, Git seems to be silently configured to use PLink without any
input from me.
Can anyone tell me why this is happening?
If I cannot choose which SSH client to use at installation, can anyone
tell me how to reconfigure Git to point to OpenSSH instead of
TortoisePLink?
This seems to controlled by a Git environment variable (mine reads
"GIT_SSH=c:\Program Files\Putty\plink.exe" as opposed to "GIT_SSH=ssh.exe"
as it does on another PC with Git working).
As a work around, does anyone know how to change this variable?