bootstrap: don't symlink source dir into stage0 sysroot

In StdLink::run we subsequently recursively copy the initial sysroot
lib directory into the stage0-sysroot lib directory.  If the initial
sysroot is a toolchain that includes the `rust-src` component (in
lib/rustlib/src/rust), if we add this symlink, that recursive copy
will overwrite the repo sources with the toolchain's sources.