PR Summary

Export VSS_NUGET_EXTERNAL_FEED_ENDPOINTS explicitly in the vPack and MSIX PowerShell build steps, including when UseEarlyAccessModules is enabled without IsEarlyAccess.

Use the same direct environment mapping as the other build templates. Keep the credential-producing variable secret and preserve the existing .NET runtime-feed-key conditions.

Add regression coverage for both build-step environment mappings and document the explicit secret-mapping requirement.

PR Context

Follow-up to #28326. Azure Pipelines does not automatically export secret variables to subsequent processes. Gating these mappings only on IsEarlyAccess leaves module-only builds without credentials for noninteractive module restores.

This fixes master first. After merge, backport the fix to release/v7.6.10, where #28355 carries the original module-feed feature.

Validation:

• Parsed both pipeline YAML files and verified direct credential mapping for all four early-access flag combinations.
• Confirmed existing runtime-key behavior and secret publication are unchanged.
• Executed the new regression-test regexes against the fixed files and original master: both fixes pass and both original files fail.
• PowerShell syntax parsing and git diff --check passed.
• The full Pester file could not run: Pester is unavailable locally and PowerShell Gallery hosts could not be resolved. Pipeline authentication requires Azure Pipelines validation.

PR Checklist

• PR has a meaningful title
• Summarized changes
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge.
• Breaking changes
  • None
  • OR
  • Experimental feature(s) needed
    • Experimental feature name(s):
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed
    • Issue filed:
• Testing - New and feature
  • N/A or can only be tested interactively
  • OR
  • Make sure you've added a new test if existing tests do not effectively test the code changed