ci: fix sizediff error when changing the LLVM version

.github/workflows/sizediff-install-pkgs.sh

@@ -0,0 +1,12 @@
+# Command that's part of sizediff.yml. This is put in a separate file so that it
+# 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-16 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 \
+    llvm-16-dev \
+    clang-16 \
+    libclang-16-dev \
+    lld-16