Fix artifacts index job by downgrading awscli (#3805)

Similarly to #3803, artifact index update fails because the R2 doesn't
like the new awscli. The regression apparently comes from 1.37.0 as
well, so pin to version older than that.

.github/workflows/artifacts-index.yaml

@@ -49,7 +49,7 @@ jobs:
           python-version: ${{ env.PYTHON_VERSION }}
 
       - name: Install AWS CLI
-        run: pip install awscli
+        run: pip install 'awscli<1.37.0'
 
       - name: Create build index
         env: