chore: add workflow_dispatch trigger to build-llvm

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 16bit-ykiko

.github/workflows/build-llvm.yml

@@ -1,6 +1,7 @@
 name: build llvm
 
 on:
+  workflow_dispatch:
   pull_request:
     # if you want to run this workflow, change the branch name to main,
     # if you want to turn off it, change it to non existent branch.