formula_auditor: use brewed curl for homepage check when needed

[iMichka]

If the download url is using brewed curl, use that too for the homepage check.
Ideally one would introduce the "using: :homebrew_curl" stanza for the homepage
line in the DSL, but it is easier to implicitely use the same logic
for homepages and url's and define the "using:" only once.

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[BrewTestBot]

Review period will end on 2021-08-23 at 08:17:57 UTC.

[BrewTestBot]

Review period skipped due to critical label.

[MikeMcQuaid]

Seems like a bit of a hack but I'm happy with this to unblock us for now. We'll probably need to eventually handle the "it's needed for homepage but not download URL" case but that can wait for now.