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

## 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:

- https://github.com/dotnet/installer/pull/20903: `Source-build updates for the .NET / 8.0.129 July 2026 release.`
- https://github.com/dotnet/sdk/pull/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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