Rebootstrap pipeline targets source branch of VMR build

[ellahathaway]

When attempting to rebootstrap the main branch using the most recent build of the P7 branch, the re-bootstrap pipeline set the target branch to the source branch from the associated VMR build, which was release/10.0.1xx-preview7. As a result, the resulting PR targeted release/10.0.1xx-preview7 instead of main. Manual intervention was required to properly target main.

We should determine if we should adjust the pipeline to always target main, and/or if we should allow override of the target branch via a pipeline parameter.

[MichaelSimons]

Fixed w/https://dev.azure.com/dnceng/internal/_git/dotnet-release/pullrequest/51856