Switch to tagged version of GH action for source code checkout

It is better to stick to a latest tagged version, that considered as
stable, than to a current development version.

Author: ligurio

.github/workflows/test.yml

@@ -16,7 +16,7 @@ jobs:
             coveralls: true
     runs-on: [ubuntu-latest]
     steps:
-      - uses: actions/checkout@master
+      - uses: actions/checkout@v2
       - uses: tarantool/setup-tarantool@v1
         with:
           tarantool-version: ${{ matrix.tarantool }}