.github/workflows/rake.yml in pdd-0.21.2 vs .github/workflows/rake.yml in pdd-0.21.3

@@ -10,10 +10,10 @@
 jobs:
   test:
     name: test
     strategy:
       matrix:
-        os: [ubuntu-latest]
+        os: [ubuntu-20.04]
         ruby: [2.7]
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-ruby@v1