Use a conditional environment

Author: mcmire

.github/workflows/build-storybook.yml

@@ -6,16 +6,17 @@ on:
       STORYBOOK_TOKEN:
         required: true
 
+env:
+  # For a `pull_request` event, the branch is `github.head_ref``.
+  # For a `push` event, the branch is `github.ref_name`.
+  BRANCH: ${{ github.head_ref || github.ref_name }}
+
 jobs:
   build-storybook:
     name: Build storybook
     runs-on: ubuntu-latest
     timeout-minutes: 30
-    environment: default-branch
-    env:
-      # For a `pull_request` event, the branch is `github.head_ref``.
-      # For a `push` event, the branch is `github.ref_name`.
-      BRANCH: ${{ github.head_ref || github.ref_name }}
+    environment: ${{ env.BRANCH == 'main' && 'default-branch' }}
     steps:
       - name: Checkout and setup high risk environment
         uses: MetaMask/action-checkout-and-setup@v1