Add missing compiler_git_ref in a trunk workflow

Due to this missing `compiler_git_ref` setup, cache is not properly
invalidated on `trunk` updates

Author: shym

.github/workflows/windows-510-trunk-bytecode-workflow.yml

@@ -8,4 +8,5 @@ jobs:
     with:
       runs_on: windows-latest
       compiler: ocaml.5.1.0,ocaml-option-mingw,ocaml-option-bytecode-only
+      compiler_git_ref: refs/heads/trunk
       timeout: 360