Source-build release PR body omits runtime and uses the agent date

[lbussell]

Description

The source-build release-day PR template expects both $RUNTIME_VERSION and $RELEASE_DATE, but eng/pipeline/source-build-release/steps/re-bootstrap.yml only exports SDK_VERSION and derives RELEASE_DATE from the agent's current date.

This produces PR bodies with a missing runtime version:

• .NET Source-Build 8.0.129 July 2026 Updates installer#20903: Source-build updates for the .NET / 8.0.129 July 2026 release.
• .NET Source-Build 9.0.119 July 2026 Updates sdk#55275: Source-build updates for the .NET / 9.0.119 July 2026 release.

The 2026-08B test runs also generated July titles because they ran on July 31 even though their releaseDate parameter was 2026-08-11:

• .NET 8 test run: https://dev.azure.com/dnceng/internal/_build/results?buildId=3036151
• .NET 9 test run: https://dev.azure.com/dnceng/internal/_build/results?buildId=3036152

Root cause

source-build-release-pr.md uses $RUNTIME_VERSION and $RELEASE_DATE. The Open PR step in re-bootstrap.yml never exports RUNTIME_VERSION and sets RELEASE_DATE=$(date +"%B %Y") instead of using the pipeline's expected release date.

Suggested fix

Pass the existing runtimeVersion job variable and pipeline releaseDate parameter through source-build-base.yml -> stages/public-release.yml -> jobs/public-release-sdk.yml -> steps/re-bootstrap.yml, then export both values before envsubst. Add a focused check that renders the PR template with a future release date and a runtime version.

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