api: don't refresh cache mtime after failed JSON download

When fetch_json_api_file's curl_download raised ErrorDuringExecution
and fell back to the cached version, the code unconditionally touched
the cache file with the current mtime. On the next `brew upgrade`,
`skip_download?` saw a 'fresh' mtime and short-circuited the download
entirely, so brew kept reusing the stale formula.jws.json /
cask.jws.json until `brew cleanup` removed it.

Track `download_succeeded` and only refresh the cache mtime after a
successful curl revalidation/download. Reset the flag at the top of
the retried outer `begin` so a JSON::ParserError-triggered retry
cannot inherit a stale `true` from the previous attempt.

Closes #22089