build: manually invoke postinstall for circleci (#8466)

Author: jelbourn

.circleci/config.yml

@@ -31,6 +31,9 @@ jobs:
           key: material2-{{ .Branch }}-{{ checksum "package-lock.json" }}
 
       - run: bazel run @nodejs//:npm install
+      # For some reason, circleci needs the postinstall to be run explicitly.
+      # This may be unnecessary once ngcontainer uses nodejs 8
+      - run: bazel run @nodejs//:npm run postinstall
       - run: bazel build src/cdk/...
       - save_cache:
           key: material2-{{ .Branch }}-{{ checksum "package-lock.json" }}