Try giving upload job write pages permission

See https://stackoverflow.com/questions/72504998/github-actions-unable-to-get-actions-id-token-request-url-env-variable
and https://docs.github.com/en/actions/security-for-github-actions/security-guides/automatic-token-authentication#permissions-for-the-github_token

.github/workflows/sphinx-using-conda.yml

@@ -55,6 +55,10 @@ jobs:
         # Upload entire HTML pages folder
         path: build/html/
   deploy:
+    permissions:
+      contents: read
+      pages: write
+      id-token: write
     environment:
       name: github-pages
       url: ${{ steps.deployment.outputs.page_url }}