[CICD] Pin the version of ubuntu image to 20.04.

.github/workflows/ubuntu_basic.yml

@@ -10,23 +10,24 @@ jobs:
   run-unit-tests:
     timeout-minutes: 60
     runs-on: ubuntu-latest
-    container: docker.io/library/ubuntu:latest
+    container: docker.io/library/ubuntu:20.04
 
     steps:
     - uses: actions/checkout@v2
 
     - name: Install bazel
       run:  |
             apt-get update
-            apt-get install -yq wget gcc g++ python3 zlib1g-dev zip libuv1.dev
+            apt-get install -yq wget gcc g++ python3.8 zlib1g-dev zip libuv1.dev
+
             apt-get install -yq pip
             wget "https://github.com/bazelbuild/bazel/releases/download/5.1.0/bazel_5.1.0-linux-x86_64.deb" -O bazel_5.1.0-linux-x86_64.deb
             dpkg -i bazel_5.1.0-linux-x86_64.deb
 
     - name: Install dependencies
       run:  |
             python3 -m pip install virtualenv
-            python3 -m virtualenv -p python3 py3
+            python3 -m virtualenv -p python3.8 py3
             . py3/bin/activate
             which python
             pip install pytest torch