Update mac builder to medium gen2 resource class (emscripten-core#18987)

see https://circleci.com/product/features/resource-classes/
The 'medium' class will go away later this year. The
gen2 class is more expensive per build minute but according to my test
https://app.circleci.com/pipelines/github/emscripten-core/emscripten/26604/workflows/c2acf4d1-1d2d-4556-b148-32278b852d42/jobs/614937
vs
https://app.circleci.com/pipelines/github/emscripten-core/emscripten/26608/workflows/8ecccca3-44b6-493e-a3e8-62e74f931fda/jobs/615017
it's enough faster that it's slightly cheaper overall.

Author: dschuff

.circleci/config.yml

@@ -25,6 +25,7 @@ executors:
       EMSDK_NOTTY: "1"
     macos:
       xcode: "13.4.1"
+    resource_class: macos.x86.medium.gen2
 
 commands:
   download-chrome: