[CI] Use ubuntu-20.04 image

Using ubuntu-latest still causes long delays due to missing runners.

Author: bader

.github/workflows/sycl_gen_test_matrix.yml

@@ -46,7 +46,7 @@ on:
 jobs:
   test_matrix:
     name: Generate Test Matrix
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     outputs:
       lts_matrix: ${{ steps.work.outputs.lts_matrix }}
       cts_matrix: ${{ steps.work.outputs.cts_matrix }}