ci: update apt repo for sizediff toolchain

This was still at jammy (22.04), while the CI container was noble
(24.04). Somehow this didn't break, but it certainly isn't ideal to
install packages across Ubuntu versions!

.github/workflows/sizediff.yml

@@ -9,6 +9,8 @@ concurrency:
 
 jobs:
   sizediff:
+    # Note: when updating the Ubuntu version, also update the Ubuntu version in
+    # sizediff-install-pkgs.sh
     runs-on: ubuntu-24.04
     permissions:
       pull-requests: write