In bootstrap, we set `RUSTC_INSTALL_BINDIR` to `config.bindir`, so
rustdoc can find rustc relative to the toolchain sysroot. However, if a
distro script like Fedora's `%configure` sets an absolute path, then
rustdoc's `sysroot.join(bin_path)` ignores that sysroot altogether.

That would be OK once the toolchain is actually installed, but it breaks
the in-tree doc tests during the build, since `/usr/bin/rustc` is still
the old version. So now we try to make `RUSTC_INSTALL_BINDIR` relative
to the sysroot prefix in the first place.