Rollup merge of #130536 - cuviper:rustbook-dylib-path, r=Mark-Simulacrum

bootstrap: Set the dylib path when building books with rustdoc

The library path is needed when the toolchain has been configured with
`[rust] rpath = false`. Otherwise, building the reference book will get
an error when it tries to run rustdoc, like:

    rustdoc: error while loading shared libraries: librustc_driver-2ec457c3b8826b72.so

Author: workingjubilee