[release/8.0] Migrate winforms-pr internal-feed auth off retired PAT to System.AccessToken

[missymessa]

Summary

The winforms-pr pipeline (eng/pipelines/build-PR.yml) authenticates internal darc-int-* Azure Artifacts feeds during Arcade SDK toolset restore using the dn-bot-dnceng-artifact-feeds-rw PAT. That PAT was removed from the arcade secret manifest (arcade #17079, merged 2026-07-08) and expired 2026-07-11, so internal release/8.0 builds now fail with HTTP 401 (Unauthorized) during artifacts/toolset/restore.proj.

This swaps the credential to $(System.AccessToken) (build identity), mirroring what eng/pipelines/build.yml (the official CI pipeline) already does on this same branch.

Root cause / evidence

• Failing build: winforms-pr on internal/release/8.0 build 3029632 (2026-07-23) — 401 (Unauthorized) on darc-int-* feeds fetching Microsoft.DotNet.Arcade.Sdk.
• Regression window: this pipeline succeeded through 2026-07-07, then began failing 2026-07-16 onward — immediately after the PAT expired.
• The sibling CI file eng/pipelines/build.yml on release/8.0 was already migrated to $(System.AccessToken) (which is why dotnet-winforms CI passes on the same commit); only build-PR.yml was missed. release/9.0 and main are unaffected.

Verification

Applied this exact change to a test branch off internal/release/8.0 and queued the winforms-pr pipeline: build 3032700 succeeded (all Windows Debug/Release x64/x86/arm64 legs), the Setup Private Feeds Credentials step passed, and the internal-feed restore completed with no 401.

Change

eng/pipelines/build-PR.yml — Token: $(dn-bot-dnceng-artifact-feeds-rw) → Token: $(System.AccessToken)

Tracked by AzDO #11679.

Microsoft Reviewers: Open in CodeFlow