Source-indexer pipeline acquires it's own runtime

[ericstj]

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.