#1110 actions fix ubuntu version

Author: yegor256

.github/workflows/codecov.yml

@@ -6,7 +6,7 @@ name: codecov
       - master
 jobs:
   codecov:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-java@v1

.github/workflows/mvn.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [windows-latest, ubuntu-latest]
+        os: [windows-latest, ubuntu-20.04]
         java: [1.8, 11, 13]
     steps:
       - uses: actions/checkout@v2