test: Check if we can update source branch name

[VaclavElias]

This issue: Stride API Docs pages' source links are pointing into incorrect (non-existing) branch release.

According this https://github.com/dotnet/docfx/blob/main/docs/reference/docfx-environment-variables-reference.md#L13-L20 we should be able to override branch name.

We might be able to simplify also PowerShell script.

- name: Build API Docs with docfx
  env:
    DOCFX_SOURCE_REPOSITORY_URL: https://github.com/stride3d/stride  # ← The Stride source repo
    DOCFX_SOURCE_BRANCH_NAME: master                                  # ← The correct branch in Stride repo
  run: dotnet tool run docfx docfx.json