Fix CI: add llvm-config to build and disable Jekyll in gh-pages

Author: cs01

.github/workflows/test-null-safety.yml

@@ -57,7 +57,7 @@ jobs:
         run: |
           export PATH="/usr/lib/ccache:/usr/local/opt/ccache/libexec:$PATH"
           cd build
-          ninja clang FileCheck count not
+          ninja clang FileCheck count not llvm-config
 
       - name: Run strict-nullability tests
         run: |