* use the same hash function as the rest of the steps
* fix race condition due to a macOS oddity.
* fix race condition due to file truncation (rename into place instead)
* integrate with marking Step.result_cached. check if the file already
exists with fs.access before doing anything else.
* use a directory so that the file basename can be "options.zig"
instead of a hash digest.
* better error reporting in case of file system failures.
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