Read the cache in main build if it's a (Intermediate)merge branch. (#15330)

### Description
In merge branch,  the run only reads the cache generated in main build.
As a result, each run in merge branch will not upload new cache except
at the first time.

### Motivation and Context
1.Reduce the cache storage.
If there's some big changes, devs should trigger the specific builds
manually in https://dev.azure.com/onnxruntime/onnxruntime/_build. It
still reads own branch cache.

tools/ci_build/github/azure-pipelines/templates/mac-cpu-packing-jobs.yml

@@ -92,9 +92,13 @@ jobs:
       parameters:
         CacheDir: $(PROTO_CACHE_DIR)
 
+  # If it's a merge build, the run will read the cache generated by main build.
   - task: Cache@2
     inputs:
-      key: ' "$(TODAY)" | ccache  | onnxruntime | "$(Build.SourceVersion)" '
+      ${{if eq(variables['Build.SourceBranchName'], 'merge')}}:
+        key: ' "$(TODAY)" | ccache  | merge '
+      ${{else}}:
+        key: ' "$(TODAY)" | ccache  | onnxruntime | $(Build.SourceVersion) '
       path: $(ORT_CACHE_DIR)
       restoreKeys: |
         "$(TODAY)" | ccache | onnxruntime