readall: check hash generation works

[Bo98]

Rationale is that we could swap brew generate-formula-api (60-90s) with this (15s) addition to brew readall.

If to_hash fails, there is likely a typo or other sort of error in the formula that would render various brew operations non-functional on that formula even among things that don't call to_hash, e.g. brew info or simply brew install, alongwith API generation itself.

In theory, a passing brew readall should now guarantee API generation works (excluding brew bugs in the command itself of course, but brew CI should continue to run brew generate-formula-api). A "readable" formula should mean you can extract all the information about the formula.

[Bo98]

See https://github.com/Homebrew/homebrew-core/actions/runs/6339913183/job/17220138593 for the latest timings after the readall and audit perf improvements. brew generate-formula-api is now the slowest part of it. This PR will bring brew readall close to the same timings as brew generate-formula-api, because it'll largely be doing the same thing. But we can then remove the cost of loading formula files twice by dropping the API generation command, for an overall speed benefit for hopefully close to zero risk.

~90s (or at least less than 2 minutes) should be ok to run in PRs again to avoid confusions like seen in Homebrew/homebrew-core#139538, and also saves precious self-hosted CI time on things that were going to fail audits anyway.

[MikeMcQuaid]

Yup, great work again @Bo98, makes sense to me and I agree with the proposed CI changes along with keeping the actual generation in Homebrew/brew.