mirror: use curl_executable instead of curl function

[ilovezfs]

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More testing shows that the curl function results in

$ brew mirror lzip
curl: (22) The requested URL returned error: 404 Not Found
Error: Failure while executing: /usr/local/opt/curl/bin/curl -q --show-error --user-agent Homebrew/1.6.9-25-g535f1fb\ (Macintosh;\ Intel\ Mac\ OS\ X\ 10.8.5)\ curl/7.60.0 --fail --progress-bar --silent --fail --output /dev/null https://api.bintray.com/packages/homebrew/mirror/lzip

in the case where the formula has never been mirrored to Bintray before.