use --time-cond when caching the formula api json

[steveh7812]

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This is to fix #13583 by using --time-cond "<date>" instead of --header "If-Modified-Since: <date>" when refreshing the cached Formula API. This prevents older versions of curl (such as the Apple-supplied curl in macOS Big Sur) from writing a zero-byte .json file. My change will also skip the mtime comparison and overwrite the cache file if the cache file is empty, in order to automatically fix this issue on affected systems.

[Rylan12]

Looks good to me! Thanks for the PR, @steveh7812!

[MikeMcQuaid]

Great work @steveh7812!