(DO NOT MERGE) Make sure this works in x86_64-gnu-llvm-18

Author: Zalathar

src/ci/github-actions/jobs.yml

@@ -85,7 +85,7 @@ pr:
   - image: mingw-check-tidy
     continue_on_error: true
     <<: *job-linux-4c
-  - image: x86_64-gnu-llvm-17
+  - image: x86_64-gnu-llvm-18
     env:
       ENABLE_GCC_CODEGEN: "1"
     <<: *job-linux-16c