FetchContent: Don't update timestamps if files don't change

The refactoring in 17e5516e (FetchContent: Invoke steps directly and avoid a separate sub-build, 2021-01-29) uses a different way of writing out the step scripts and updating time stamps when steps are executed. That inadvertently always wrote out the scripts for custom commands, even when the contents didn't change. This caused their timestamp to always be updated, resulting in those steps always being seen as out-of-date and needing to be re-executed.

ExternalProject is not susceptible to this problem because it uses file(GENERATE) to write out the script files and that only updates the file's timestamp if the contents change.

Backport: release:HEAD^2
Topic-rename: fix-IS_NEWER_THAN-usage