fix: Ensure that Platform is specified by default in Release (backport #16720)

[mergify]

GitHub Issue (If applicable): related to #16662

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the new behavior?

Building with no Platform set will use the current OS platform.

PR Checklist

Please check if your PR fulfills the following requirements:

• Docs have been added/updated which fit documentation template (for bug fixes / features)
• Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• Validated PR Screenshots Compare Test Run results.
• Contains NO breaking changes
• Associated with an issue (GitHub or internal) and uses the automatic close keywords.
• Commits must be following the Conventional Commits specification.

Other information

Internal Issue (If applicable):

---

This is an automatic backport of pull request #16720 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of ce755d4 has failed:

On branch mergify/bp/release/stable/5.2/pr-16720
Your branch is ahead of 'origin/release/stable/5.2' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit ce755d4039.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   build/test-scripts/run-netcore-mobile-template-tests.ps1

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[unodevops]

🤖 Your Docs stage site is ready! Visit it here: https://unodocsprstaging.z13.web.core.windows.net/pr-16737/index.html