Auto merge of #95440 - jyn514:error-index, r=Mark-Simulacrum

Auto merge of #95440 - jyn514:error-index, r=Mark-Simulacrum

Fix `x test src/tools/error_index_generator --stage {0,1}`

There were two fixes needed:
1. Use `top_stage` instead of `top_stage - 1`. There was a long and torturous comment about trying to match rustdoc's version, but it works better without the hard-coding than with (before it gave errors that `libtest.so` couldn't be found).
2. Make sure that `ci-llvm/lib` is added to LD_LIBRARY_PATH. Previously the error index would be unable to load LLVM for stage0 builds.

At some point we should probably have a discussion about how rustdoc stages should be numbered;
confusion between 0/1/2 has come up several times in bootstrap now. cc https://github.com/rust-lang/rust/issues/92538

Note that this is still broken when using `download-rustc = true` and `--stage 1`,
but that's *really* a corner case and should affect almost no one. `--stage {0,2}`
work fine with download-rustc.

Fixes https://github.com/rust-lang/rust/issues/80096.