cmake: Restore normalization of `-P` script to on-disk path

This was missed in commit ee83165923 (cmake: Explicitly normalize
input paths as they exist on disk, 2024-10-17, v4.0.0-rc1~597^2),
causing `CMAKE_CURRENT_LIST_FILE` in `cmake -P` scripts to be
regressed by the KWSys behavior change merged by commit e9bd437a43
(Merge branch 'upstream-KWSys' into normalize-input-paths,
2024-10-24, v4.0.0-rc1~589^2~1).

Issue: #27750