Merge pull request #406 from antmicro/mglb/FixCI

CI: Explicitly use ubuntu-20.04
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index a98124e..4ab1814 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -21,7 +21,7 @@
 jobs:
 
   Run-tests:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps: