cfl: fix randomly failed builds

GitHub action builds started to randomly fail with a reason:
"The runner has received a shutdown signal.".
The issue [1] and discussion [2] points to a workaround:
replace a "ubuntu-latest" with "ubuntu-20.04" and it started to
work. Note that the problem reproduce with "ubuntu-22.04" as well.

The patch replaces "ubuntu-latest" with "ubuntu-20.04" in CFL
workflows.

1. actions/runner-images#6709
2. https://github.com/actions/runner-images/discussions/7188

Author: ligurio

.github/workflows/cflite_batch.yml

@@ -15,7 +15,7 @@ permissions: read-all
 
 jobs:
   luajit:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix:

.github/workflows/cflite_build.yml

@@ -10,7 +10,7 @@ permissions: read-all
 
 jobs:
   luajit:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix:

.github/workflows/cflite_cron.yml

@@ -16,7 +16,7 @@ jobs:
 # are covered by fuzzing.
 
   Coverage:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v4
       - uses: ./.github/actions/setup
@@ -46,7 +46,7 @@ jobs:
 # code coverage.
 
   Pruning:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v4
       - uses: ./.github/actions/setup

.github/workflows/cflite_pr.yml

@@ -35,7 +35,7 @@ permissions: read-all
 
 jobs:
   luajit:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix: