Investigate ways to exclude llvm sources from builder image

The changes in https://github.com/dotnet/dotnet-buildtools-prereqs-docker/pull/1456 had to add back the `llvm-project.src` directory in crossdeps-builder because it's needed by derived images. There should be an investigation of ways to avoid needing this. One option is to simply require the derived image to download the source tarball itself rather than relying on it existing in the base image. This directory accounts for 1.7 GB so it's definitely significant.