Set environment variables in a CI-native way

.gitea/workflows/ci.yml

@@ -14,10 +14,10 @@ jobs:
           python-version: "3.11"
       - name: Install dependencies
         run: pip install -r requirements.txt
-      - run: |
-          export GITEA_TOKEN=$GITHUB_TOKEN
-          export GITEA_URL=$GITHUB_SERVER_URL
       - name: Build
         run: ./resize.py
       - name: Upload
         run: ./upload.py
+        env:
+          GITEA_TOKEN: ${{ env.GITHUB_TOKEN }}
+          GITEA_URL: ${{ env.GITHUB_SERVER_URL }}