Rollup merge of #142000 - lambdageek:no-symlink-stage0, r=onur-ozkan

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.

Fixes rust-lang/rust#141991

Author: matthiaskrgr