Use large macos instance (facebook#8320)

Summary:
Macos build is taking more than 1 hour, bump the instance type from the
default medium to large (large macos instance was not available before).

Pull Request resolved: facebook#8320

Test Plan: watch CI pass

Reviewed By: ajkr

Differential Revision: D28589456

Pulled By: jay-zhuang

fbshipit-source-id: cff78dae5aaf9de90ade3468469290176de5ff32

.circleci/config.yml

@@ -121,6 +121,7 @@ jobs:
   build-macos:
     macos:
       xcode: 11.3.0
+    resource_class: large
     steps:
       - increase-max-open-files-on-macos
       - install-pyenv-on-macos
@@ -132,6 +133,7 @@ jobs:
   build-macos-cmake:
     macos:
       xcode: 11.3.0
+    resource_class: large
     steps:
       - increase-max-open-files-on-macos
       - install-pyenv-on-macos