tests: pin the CI runner to ubuntu-24.04.

ubuntu-latest now resolves to Ubuntu 26.04 (resolute), for which
openresty.org does not provide an apt repository yet, so every job
failed in "Add OpenResty apt repository" with a 404 on the Release file.

Author: oowl

.github/workflows/build_and_test.yml

@@ -10,7 +10,9 @@ on:
 jobs:
   test:
     name: ${{ matrix.name }}
-    runs-on: ubuntu-latest
+    # pinned: openresty.org has no apt repository yet for the Ubuntu release
+    # ubuntu-latest now points to
+    runs-on: ubuntu-24.04
 
     strategy:
       fail-fast: false