Fix download issues in pipelines

[vmoroz]

Description

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Why

1. yarn validate-overrides hits GitHub's unauthenticated API rate limit (60 req/hr),
   causing intermittent linting failures.
2. .NET Core Hosting Bundle and SDK downloads in Desktop builds fail due to transient
   network errors (The response ended prematurely), breaking builds.

What

• Linting job (linting.yml): Pass PLATFORM_OVERRIDE_GITHUB_TOKEN to
  yarn validate-overrides to authenticate GitHub API calls (5,000 req/hr).
  • PR builds: token comes from the platform-override-zero-permission-token
    variable group (non-secret, works with fork PRs).
  • CI builds: token is extracted from the checkout OAuth credential via
    persistCredentials: true (stored as a secret pipeline variable).
• Checkout template (checkout-shallow.yml): Add optional persistCredentials
  parameter (default: false, no impact on other jobs).
• Desktop builds (desktop-single.yml): Add Invoke-WebRequestWithRetry helper
  (3 attempts, incremental backoff) for .NET hosting bundle and SDK downloads.

Screenshots

N/A — pipeline YAML changes only.

Testing

• PR pipeline: Linting job passes, validate-overrides uses token from variable group
• CI pipeline: Linting job passes, OAuth token extracted (length=40), validate-overrides succeeds
• Fork PR: Linting job passes with variable group token
• Desktop builds: .NET downloads succeed (retry logic)

Changelog

Should this change be included in the release notes: no