ci: fix version of ubuntu

More info: tarantool/tarantool#5429 (comment)

Author: 0x501D

.github/workflows/test.yml

@@ -15,7 +15,7 @@ jobs:
         include:
           - tarantool: '2.10'
             coveralls: true
-    runs-on: [ubuntu-latest]
+    runs-on: [ubuntu-20.04]
     steps:
       - uses: actions/checkout@master
       - uses: tarantool/setup-tarantool@v1