[Bug] Referencing globalMetadata in .md files

[m1st001]

Describe the bug
References to globalMetadata, fileMetadata and YAML front matter are not resolving.

To Reproduce
Steps to reproduce the behavior:

1. Minimal example:
   docfx.json and index.md are located in the same folder.
   docfx.json

{
  "$schema": "https://raw.githubusercontent.com/dotnet/docfx/main/schemas/docfx.schema.json",
  "build": {
    "content": [
      {
        "files": ["**/*.md"],
        "src": "."
      }
    ],
    "resource": [
      {
        "files": ["images/**"]
      }
    ],
    "output": "_site",
    "fileMetadata": {
      "author": {
        "**.md": "123123",
        "spec/**.md": 3
      }
    },
    "globalMetadata": {
      "_appTitle": "Sample App",
      "injection": "abc"
    }
  }
}

index.md

---
_layout: landing
---

# This is the **HOMEPAGE**. **{{_layout}}**

{{author}}

- Global `injection`: **{{injection}}**
- Global `_appTitle`: **{{_appTitle}}**

Expected behavior
I expected {{injection}} to be rendered as abc etc.

Context (please complete the following information):

• OS: Windows 11
• Docfx version: 2.78.2+3ff9ae7a9700029e0023a7ece18a723d351f1e9b

Additional context
Add any other context about the problem here.