Update generate.yaml

Author: ldetmer

.github/workflows/generate.yaml

@@ -36,6 +36,7 @@ jobs:
           sudo apt update
           sudo apt install python3
           python3 --version
+          pip install pip==21.3.1
           pip --version  
       - run: ./google-api-java-client-services/.github/workflows/generate.sh ${{ matrix.service }}
       - uses: googleapis/code-suggester@v2 # takes the changes from git directory