[request] Restore timestamps when restoring from cache.

I have set up wireit in CI (Azure DevOps) to improve build times. When wireit runs it will always restore files from cache since there are no output folders. Suprisingly this leads to the `manifest` files having diffs where I am expecting no changes.

Judging by the code this is intentional as no timestamps are restored. This makes sense from the "local machine" workflow but when the output folders are _always_ restored from cache (i.e. external clean, before build) this seems odd. I half expect cache restoration to preserve timestamps.

I suggest that an additional `timestamp` file is stored in the cache folder of a script and that `fs.utimes` is used to set the modified time when restoring from cache. This will more closely match a "restored" state. It also allows other tools to more accurately gauge if files restored from change or not.