ci(circleci): prevent timeout on circle-ci macos node10 build (#7651)

The circle ci job for macOS on node 10 currently fails because it takes too long
(both PR's #7649 and #7650 don't green because of that). A big chunk of the time
is taken by updating homebrew - which in this macOS (the one connected to xcode 9
=> macOS 10.12) does not contain node 10 by default.

This change:
- sets the required xcode version to 11.0 -> this gets us a more up to date macOS
  version (except for node 6, which isn't available in the homebrew associated
  with xcode 11)
- removes the brew update from the macOS node 10 job

.circleci/config.yml

@@ -7,7 +7,7 @@ docker_defaults: &docker_defaults
 
 macos_defaults: &macos_defaults
   macos:
-    xcode: "9.0"
+    xcode: "11.0"
   working_directory: ~/project/yarn
 
 attach_workspace: &attach_workspace
@@ -155,7 +155,6 @@ jobs:
           name: Install Node 10
           command: |
             brew uninstall --ignore-dependencies node
-            brew update
             HOMEBREW_NO_AUTO_UPDATE=1 brew install node@10
             brew link --overwrite --force node@10
             [[ $(node --version) =~ ^v10\. ]]
@@ -177,6 +176,8 @@ jobs:
       - *test_run
   test-macos-node6:
     <<: *macos_defaults
+    macos:
+      xcode: "9.0"
     steps:
       - run:
           name: Install Node 6