[Caching] Use FileSystem::writeAtomic() in FileCacheStorage to avoid partial cache reads on parallel run

[TomasVotruba]

Alternative to #8426, following the suggestion to drop the hand-rolled workarounds and reuse Nette\Utils\FileSystem::writeAtomic() (added in nette/utils 4.1.5, cc0326d).

Problem

On parallel runs, save() used copy() to publish the cache file. copy() truncates the destination and streams into it, so it is not atomic - a concurrent worker that require()s the same path during boot can read a half-written file and crash:

{"fatal_errors":["syntax error, unexpected string content ..."]}
 [ERROR] Could not process some files, due to: "Child process error".

This hits Linux too, not only Windows.

Change

• save() now calls FileSystem::writeAtomic(), which writes to a temp file and rename()s it into place - an atomic replace, so a reader always sees either the old or the new complete file. It also carries the Windows retry loop for the rename() edge case that motivated the original copy() switch (Could not process a file due to System error: Cannot rename rector#8432, rename fails on Windows PHP 8.1 if the target file is being executed php/php-src#7910), so no separate workaround is needed.
• Bumped nette/utils to ^4.1.5 (first release with writeAtomic()).
• Added a regression test that pins the precondition: a reader that opened the file before a save() must still see the complete contents it opened, and fails the moment copy() is put back.

Fixes rectorphp/rector#9876