Re-disabled brew update (rust-lang#630)

Author: zhassan-aws

scripts/setup/macos-10.15/install_deps.sh

@@ -7,7 +7,7 @@ set -eux
 # Github promises weekly build image updates, so we can skip the update step and
 # worst case we should only be 1-2 weeks behind upstream brew.
 # https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners#supported-software
-brew update
+#brew update
 
 # Install dependencies via `brew`
 brew install ctags