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-install-pkgs.sh

@@ -2,7 +2,7 @@
 # still works after checking out the dev branch (that is, when going from LLVM
 # 16 to LLVM 17 for example, both Clang 16 and Clang 17 are installed).
 
-echo 'deb https://apt.llvm.org/jammy/ llvm-toolchain-jammy-18 main' | sudo tee /etc/apt/sources.list.d/llvm.list
+echo 'deb https://apt.llvm.org/noble/ llvm-toolchain-noble-18 main' | sudo tee /etc/apt/sources.list.d/llvm.list
 wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
 sudo apt-get update
 sudo apt-get install --no-install-recommends -y \

.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