Fix homebrew (#8791)

The macOS tests run on this PR would fail without this change.

* brew update

* add link to upstream issue

.azure-pipelines/templates/steps/tox-steps.yml

@@ -1,6 +1,10 @@
 steps:
+  # We run brew update because we've seen attempts to install an older version
+  # of a package fail. See
+  # https://github.com/actions/virtual-environments/issues/3165.
   - bash: |
       set -e
+      brew update
       brew install augeas
     condition: startswith(variables['IMAGE_NAME'], 'macOS')
     displayName: Install MacOS dependencies