cmake: Always update timestamps on test and install scripts

Since commit d46d8df0ed (ENH: Re-implemented cmGeneratedFileStream to
... replace the destination file atomically, 2004-11-03, v2.4.0~2578) we
have generated `cmake_install.cmake` scripts with "copy-if-different"
semantics such that their timestamp is only updated when their contents
change.  In commit d395b563ed (ENH: ... test files are written by local
generator, 2005-04-24, v2.4.0~1842) we copied the approach for
generating `CTestTestfile.cmake` scripts.

In both cases, preserving timestamps is not needed and was only a side
effect of using `cmGeneratedFileStream` to replace the generated files
atomically.  Drop the "copy-if-different" semantics and always update
their timestamps so it is possible to know when they have been
regenerated.