system-libs default [.NET 10]

Currently the vmr defaults to build with bundled libs.

To fix an issue in .NET 9 we switched to use system brotli in https://github.com/dotnet/runtime/pull/107225 and in https://github.com/dotnet/runtime/pull/107166 we're switching back to bundled for .NET 10.

If there is a set of libraries that maintainers will likely prefer to use from the system, it would be nice if the vmr would default to those for "non-portable, source-only" builds.

I assume that brotli is one of those because there was zero objection to switch to the system brotli in https://github.com/dotnet/runtime/issues/107020.

cc @MichaelSimons @omajid @jkotas @dotnet/distro-maintainers