Source-indexer pipeline acquires it's own runtime

Related https://github.com/dotnet/arcade/issues/7747

There's a hardcoded `major.minor` version of the SDK referenced https://github.com/dotnet/arcade/blob/aec4b170fd6d0490f12c4b51607f55df3622076e/eng/common/core-templates/steps/source-index-stage1-publish.yml#L9-L14 for running the source-indexer tools. Rather than doing this and keeping it up to date, we should just use the SDK that the repo build _already acquires_ and keeps up to date.

This avoids a hardcoded version in the arcade build scripts that needs to be kept up to date and it also ensures that we are more likely to use the version of components (MSBuild/Roslyn) used to build the repo.

Start with eng/common/core-templates/steps/source-index-stage1-publish.yml at the linked lines and read the related Arcade issue 7747. Trace how the repository build acquires its SDK, then verify the source-indexer pipeline uses that SDK rather than a hardcoded major.minor version and still runs successfully.

Written by the indexing model from the issue text.