use v2 checkout in all workflows

Author: nulano

.github/workflows/lint.yml

@@ -13,7 +13,7 @@ jobs:
     name: Python ${{ matrix.python-version }}
 
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2
 
     - name: pip cache
       uses: actions/cache@v1

.github/workflows/test-docker.yml

@@ -29,7 +29,7 @@ jobs:
     name: ${{ matrix.docker }}
 
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2
 
     - name: Build system information
       run: python .github/workflows/system-info.py