Minor updates to CI configuration (#15458)

* Check on Python versions uses action/checkout@v2
* Update the apt cache before installing system packages

.github/workflows/linux_build_tests.yaml

@@ -42,6 +42,7 @@ jobs:
         python-version: 3.8
     - name: Install System Packages
       run: |
+        sudo apt-get update
         sudo apt-get -yqq install ccache gfortran perl perl-base r-base r-base-core r-base-dev findutils openssl libssl-dev libpciaccess-dev
         R --version
         perl --version

.github/workflows/minimum_python_versions.yaml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2
     - name: Setup Python
       uses: actions/setup-python@v1
       with: