ci: Fix build-storybook workflow again

There are still a couple of issues with the `build-storybook` workflow
that prevent it from running effectively.

1. The environment is incorrectly named (should be `default-branch`).
   I've fixed this.
2. The `deploy-storybook` job is not running even on pushes to `main`.
   It seems that we are checking for the existence of a
   `STORYBOOK_TOKEN` environment variable when we should be checking for
   the secret. Although in practice, the secret has been required this
   whole time so the conditional seems redundant. I've corrected this by
   making the secret optional.

Author: mcmire

.github/workflows/build-storybook.yml

@@ -4,14 +4,14 @@ on:
   workflow_call:
     secrets:
       STORYBOOK_TOKEN:
-        required: true
+        required: false
 
 jobs:
   build-storybook:
     name: Build storybook
     runs-on: ubuntu-latest
     timeout-minutes: 30
-    environment: ${{ github.ref_name == 'main' && 'default_branch' || null }}
+    environment: ${{ github.ref_name == 'main' && 'default-branch' || null }}
     env:
       # For a `pull_request` event, the branch is `github.head_ref``.
       # For a `push` event, the branch is `github.ref_name`.
@@ -37,7 +37,7 @@ jobs:
           path: storybook-build
 
       - name: Deploy storybook
-        if: ${{ env.BRANCH == 'main' && env.STORYBOOK_TOKEN }}
+        if: ${{ env.BRANCH == 'main' && secrets.STORYBOOK_TOKEN }}
         env:
           STORYBOOK_TOKEN: ${{ secrets.STORYBOOK_TOKEN }}
         run: |