tools: have test-asan use ubuntu-20.04

PR-URL: #45581
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

Author: panva

.github/workflows/test-asan.yml

@@ -23,7 +23,7 @@ env:
 
 jobs:
   test-asan:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     env:
       CC: clang
       CXX: clang++