ci: Derive deploy environment from the branch ref

Select the environment from github.ref rather than the event type, so a
workflow_dispatch off a feature branch resolves to preview instead of
production. Production is only reached from the default branch.

Author: DaveSkender

.github/workflows/deploy-website.yml

@@ -24,7 +24,7 @@ jobs:
       JEKYLL_GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
     environment:
-      name: stockindicators.dev
+      name: ${{ github.ref == 'refs/heads/main' && 'stockindicators.dev' || 'preview' }}
       url: ${{ steps.deploy.outputs.pages-deployment-alias-url }}
 
     steps: