[release/13.4] Remove Corepack from extension build scripts

[joperezr]

Description

Reverts the Corepack-specific Yarn invocation from the 13.4 branch VS Code extension build scripts.

Internal Azure Pipelines builds run with network isolation, and corepack yarn@1.22.22 attempts to download Yarn from the public Yarn registry before the extension's .yarnrc package registry configuration is used. That public download is blocked in internal builds. For 13.4, use the lower-risk fix of invoking the configured yarn executable directly again while preserving the Yarn-only/frozen-lockfile restore path.

Validation:

• git diff --check
• Parsed extension\Extension.proj as XML
• Confirmed no corepack references remain in the targeted extension build files

Fixes # (issue)

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
  • No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • Yes
  • No
• Did you add public API?
  • Yes
    • If yes, did you have an API Review for it?
      • Yes
      • No
    • Did you add <remarks /> and <code /> elements on your triple slash comments?
      • Yes
      • No
  • No
• Does the change make any security assumptions or guarantees?
  • Yes
    • If yes, have you done a threat model and had a security review?
      • Yes
      • No
  • No

[github-actions]

🚀 Dogfood this PR with:

⚠️ WARNING: Do not do this without first carefully reviewing the code of this PR to satisfy yourself it is safe.

curl -fsSL https://raw.githubusercontent.com/microsoft/aspire/main/eng/scripts/get-aspire-cli-pr.sh | bash -s -- 17628

Or

• Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/microsoft/aspire/main/eng/scripts/get-aspire-cli-pr.ps1) } 17628"

[joperezr]

Merging to unblock the intenral build.

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