ci: limit parallelism for Ember builds (#7917)

If not artificially limited by setting JOBS env var,
broccoli-babel-transpiler will attempt to parallelize across the number
of CPUs on the host VM rather than the Docker container, resulting in CI
jobs being killed due to running out of memory.

.circleci/config.yml

@@ -411,7 +411,8 @@ jobs:
   ember-build:
     docker:
       - image: *EMBER_IMAGE
-    resource_class: medium+
+    environment:
+      JOBS: 2 # limit parallelism for broccoli-babel-transpiler
     steps:
       - checkout
       - restore_cache:
@@ -428,7 +429,8 @@ jobs:
   ember-build-prod:
     docker:
       - image: *EMBER_IMAGE
-    resource_class: medium+
+    environment:
+      JOBS: 2 # limit parallelism for broccoli-babel-transpiler
     steps:
       - checkout
       - restore_cache: