[ci] Pin CI to LLVM 12 for clang_12 branch
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index f344df4..1f06014 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -15,7 +15,7 @@
       fail-fast: false
 
     env:
-      LLVM_TAG:
+      LLVM_TAG: -12
 
     steps:
       - name: Install prerequisites