In 315a200f (FindGit: Cache the GIT_EXECUTABLE version for the
current run, 2021-01-20), the GIT_VERSION_STRING was meant to be
cached after the first time it was computed for a given GIT_EXECUTABLE
location. That logic assumed GIT_VERSION_STRING would be visible in
the current scope, but it might not be. The global property alone is
enough to check whether the version has been determined previously,
so don't switch the logic based on whether GIT_VERSION_STRING is
defined or not.

Relates: #21703