Rollup merge of #52594 - Mark-Simulacrum:error-index-stage0, r=alexcrichton

Run the error index tool against the sysroot libdir

Previously when building the error index tool in stage 0 we would
attempt to use stage 0 libraries, but because it depends on rustdoc,
those don't exist: rustdoc is built against stage 1 libraries.

This patch aligns those two and passes the stage 1 libdir to the error
index.

@GuillaumeGomez discovered that this hasn't worked (presumably for a long time now, but not sure).

r? @alexcrichton