Update LLVM version to 21.1.8 in build.yml

Author: alexreinking

.github/workflows/build.yml

@@ -31,7 +31,7 @@ jobs:
           # - { major: 18, full: 18.1.8 }
           # - { major: 19, full: 19.1.7 }
           # - { major: 20, full: 20.1.8 }
-          - { major: 21, full: 21.1.1 }
+          - { major: 21, full: 21.1.8 }
     steps:
       - uses: actions/checkout@v4