Fix shellcheck warning in Playwright workflow

Use env: block for RAILS_ENV instead of inline variable assignment
to satisfy shellcheck linting requirements.

Changes:
- Move RAILS_ENV=test to env: block in Generate React on Rails packs step
- Fixes SC2209 shellcheck warning

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: justin808

.github/workflows/playwright.yml

@@ -40,7 +40,9 @@ jobs:
 
       - name: Generate React on Rails packs
         working-directory: spec/dummy
-        run: RAILS_ENV=test bundle exec rake react_on_rails:generate_packs
+        env:
+          RAILS_ENV: test
+        run: bundle exec rake react_on_rails:generate_packs
 
       - name: Build test assets
         working-directory: spec/dummy