[release pipeline] Retain MSFT pipeline runs used in 8.0/9.0 releases

[ellahathaway]

Related to #3700

I've observed that the msft pipeline runs referenced in source-build releases for 8.0 and 9.0 are not retained. Once those pipeline runs are cleaned up, release validation fails. We should consider retaining the msft pipeline runs to avoid this issue.

This also doesn't apply to referenced VMR builds, since we currently retain those. Similarly, this does not affect 10.0 and later, where msft builds align with source-build builds and are produced out of the VMR. In 8.0 and 9.0, however, the msft build occurs in the installer and SDK repositories, respectively, making the retention of those pipeline runs necessary for successful validation.

[ellahathaway]

PR: https://dev.azure.com/dnceng/internal/_git/dotnet-release/pullrequest/52089