Actually pass --stable to brew test-bot in the stable tests

[issyl0]

• Have you followed the guidelines for contributing?
• Have you ensured that your commits follow the commit style guide?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
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I did a couple of PRs to enable typechecking for taps in brew test-bot. Then the tap_syntax job failed because I forgot about the stable release tag jobs. So I made the typechecking part only run if on HEAD. Then it turns out that this job wasn't actually passing --stable to brew test-bot.

[MikeMcQuaid]

Nice work @issyl0!