CI: pin the Pages build job to ubuntu-24.04

ubuntu-latest now resolves to ubuntu-26.04; ruby/setup-ruby v1.207.0 (pinned)
has no prebuilt Ruby for it and fails as if the runner were self-hosted.

Author: oguzhan-baser

.github/workflows/jekyll.yml

@@ -29,7 +29,8 @@ concurrency:
 jobs:
   # Build job
   build:
-    runs-on: ubuntu-latest
+    # Pinned: ubuntu-latest moved to 26.04, which the pinned setup-ruby below has no prebuilt Ruby for
+    runs-on: ubuntu-24.04
     steps:
       - name: Checkout
         uses: actions/checkout@v4