CI: explicitly specify the OS version when necessary

Commit 8ea120f6 added --break-system-packages which works in Ubuntu
24.04 but not 22.04, so explicitly specify that version in the runner
instead of relying on ubuntu-latest to provide it.  Some runners have
regressed back to 22.04 for ubuntu-latest, resulting in build failures.

.github/workflows/http3-linux.yml

@@ -254,7 +254,7 @@ jobs:
     needs:
       - setup
       - build-cache
-    runs-on: 'ubuntu-latest'
+    runs-on: 'ubuntu-24.04'
     timeout-minutes: 45
     strategy:
       fail-fast: false

.github/workflows/linux.yml

@@ -61,7 +61,7 @@ env:
 jobs:
   linux:
     name: ${{ matrix.build.generate && 'CM' || 'AM' }} ${{ matrix.build.name }}
-    runs-on: 'ubuntu-latest'
+    runs-on: 'ubuntu-24.04'
     container: ${{ matrix.build.container }}
     timeout-minutes: 45
     strategy: