github CI: use ubuntu-20.04 (not -latest)

affects only the lint job, everything else used 20.04 already.

.github/workflows/ci.yml

@@ -31,7 +31,7 @@ on:
 jobs:
   lint:
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     timeout-minutes: 10
 
     steps:

.github/workflows/codeql-analysis.yml

@@ -14,7 +14,7 @@ on:
 jobs:
   analyze:
     name: Analyze
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     permissions:
       actions: read
       contents: read