CMakeVersion: Prefer Git information provided by 'git archive' exports

Update the approach from commit fe2c558ba6 (CMakeVersion: Preserve Git
information during 'git archive', 2019-07-25, v3.16.0-rc1~337^2) to
prefer `export-subst` information.  This will allow exported source
trees to be imported into unrelated Git repositories while still using
the original upstream CMake commit information.
1 file changed