ci: pin runners to ubuntu-24.04 ahead of the ubuntu-latest move

.github/workflows/e2e.yml

@@ -24,7 +24,7 @@ jobs:
   # mobile route sweep landed, and a slow runner tipped them over; three
   # shards with a 30-minute cap leave room for both.
   shard:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04
     timeout-minutes: 30
     strategy:
       fail-fast: false
@@ -128,7 +128,7 @@ jobs:
   # that name and answers for both shards: it fails when any shard failed or
   # was cancelled, so a half-green matrix cannot satisfy the gate.
   e2e:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04
     needs: shard
     if: always()
     timeout-minutes: 5