ci: Attempt to make macOS builds more reliable

[real-or-random]

The macOS CI tasks often error fail when doing brew update with
git fetch errors:

remote: fatal: packfile /data/repositories/b/nw/b6/07/5c/123272362/network.git/objects/pack/pack-2139bd07361b62a358e380a0e7d58ec35593d191.pack cannot be accessed
fatal: protocol error: bad pack header
Error: Fetching /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core failed!

Superficially this seems to be a problem on the GitHub server because
the message shows a "remote" error. But it seems we're the only one in
the world running into this specific issue when doing brew update, so
it's more likely that the something else is the culprit, and this error
message is just a symptom.

This commit replaces brew update with a complete reinstallation of
brew. This is essentially a shot in the dark but it's worth a try, and
I doubt it's significantly more expensive. If that won't work, we may
consider simply retrying brew update a few times.

[real-or-random]

Ok CI passed on first try, and this adds < 10 seconds to the macOS tasks. Let's re-run it. :P

edit: ok, let's try a third a last time.

[real-or-random]

Ok, all three runs pass. I think it's worth the try and merge it.

[jonasnick]

ACK d9396a5

See issue cirruslabs/macos-image-templates#41 (comment) for more context.

[real-or-random]

Great, now we don't even get proper error messages any more: https://cirrus-ci.com/task/6275416297046016?logs=brew_valgrind_pre#L126

I think we should use the script that Core uses: https://github.com/bitcoin/bitcoin/tree/master/ci/retry