Fixes test-bot mishandling of optional cross-tap dependencies. Without
this change, since formula.stable.deps includes the optional dependencies but
formula.recursive_dependencies does not, test-bot was trying to select a
compiler for an untapped formula, which raised an error.

Our suspect handling of optional dependencies was exposed by Homebrew/homebrew#43145 /
7184348e822d6745d8a103de104ff9626536bf1e.

Closes Homebrew/homebrew#43237.