[devops] recover tsinghua pip source due to proxy issue (#509)

.github/workflows/build.yml

@@ -20,6 +20,7 @@ jobs:
     steps:
       - name: Install dependencies
         run: |
+          pip config set global.index-url https://pypi.tuna.tsinghua.edu.cn/simple
           pip install -U pip setuptools wheel --user
           pip install pytest tensorboard transformers
       - uses: actions/checkout@v2

.github/workflows/compatibility_test.yml

@@ -25,6 +25,7 @@ jobs:
     steps:
       - name: Install dependencies
         run: |
+          pip config set global.index-url https://pypi.tuna.tsinghua.edu.cn/simple
           pip install -U pip setuptools wheel --user
           pip install pytest tensorboard deepspeed apex
       - uses: actions/checkout@v2