.github: use improved checkout action

.github/workflows/main.yml

@@ -6,7 +6,7 @@ jobs:
   android-arm:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2
     - name: prepare
       run: |
         sudo apt-get update
@@ -29,7 +29,7 @@ jobs:
   linux:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2
     - name: prepare
       run: |
         sudo apt-get update
@@ -64,7 +64,7 @@ jobs:
   windows:
     runs-on: windows-latest
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2
     - name: luajit
       shell: cmd
       run: |