Tarball should build with whatever SDK is in the .dotnet/ dir, without specifying --with-sdk

Currently, if you extract an SDK to the `.dotnet/` directory in a tarball and it doesn't match the SDK version found in global.json, the tarball build fails. The workaround is to add the `--with-sdk .dotnet/` argument to the build, which causes the build script to automatically ask the SDK which version it is: https://github.com/dotnet/installer/blob/release/7.0.1xx/src/SourceBuild/tarball/content/build.sh#L119

We even do this for our own bootstrap builds: https://github.com/dotnet/installer/blob/3c4322d8e5f92a3a2a2d51095c2f06bb1c033b88/src/SourceBuild/Arcade/eng/common/templates/job/source-build-build-tarball.yml#L44

I think that we should always ask the installed SDK what version it is and use that. You should only have to specify `--with-sdk` when building with an SDK outside of the tarball's `.dotnet/` directory.