assemble-mac - Fix Apple certificate signing issue (#112)

## What is the goal of this PR?
The Apple Certificate signing in the `assemble-mac` CI job is broken. This PR fixes that

## What are the changes implemented in this PR?
Define the environment variables containing the certificate appropriately

.circleci/config.yml

@@ -105,9 +105,11 @@ jobs:
     steps:
       - install-bazel-mac
       - checkout
-      - run: CSC_LINK="$APPLE_CODE_SIGNING_CERTIFICATE_URL" CSC_KEY_PASSWORD="$APPLE_CODE_SIGNING_PASSWORD"
       - run: bazel run @nodejs//:npm install
-      - run: bazel run @nodejs//:npm run build
+      - run: |
+          export CSC_LINK="$APPLE_CODE_SIGNING_CERTIFICATE_URL"
+          export CSC_KEY_PASSWORD="$APPLE_CODE_SIGNING_PASSWORD"
+          bazel run @nodejs//:npm run build
       - run: mkdir -p ~/distribution && mv ./build/*.dmg ~/distribution/grakn-workbase-mac-$(cat VERSION).dmg
       - persist_to_workspace:
           root: ~/distribution