ci(macos): skip brew upgrade

Problem: Support for Intel macOS is increasingly being dropped since
Apple itself retired the architecture (with macOS 27 already being
ARM-only). In particular, homebrew is already dropping support, with
some 3rd-party taps like `hashicorp/tap/vagrant` (that are enabled on GH
runners by default) no longer being available. This makes `brew upgrade`
fail on Intel.

Solution: Skip `brew upgrade` in `install_deps.sh`, as we don't rely on
the bleeding edge version of the installed dependencies. This skips
checking the tap (which we don't use) and hence lets the workflows pass.
(It also shaves of some seconds from the CI time.)

Note: This is strictly speaking a GHA issue and should be handled by
adapting the runner images; but since GHA has already announced that
they will drop all Intel macOS images in 2027, this is unlikely to
happen.

It also means that this is a stopgap solution to allow 0.13 to keep Tier
1 support for Intel macOS, but next year we'll have to drop support as
well.

.github/scripts/install_deps.sh

@@ -49,7 +49,6 @@ if [[ $OS == Linux ]]; then
     fi
   fi
 elif [[ $OS == Darwin ]]; then
-  brew update --quiet
   if [[ -n $TEST ]]; then
     brew install cpanminus fish fswatch