autodiff: fix sysroot candidates in libEnzyme not found error

[sgasho]

fix: #160335 (comment)

Before

RUSTFLAGS="-Zautodiff=Enable" cargo +enzyme run
info: `cargo` is unavailable for the active toolchain
info: falling back to "/home/gh-sgasho/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo"
error: failed to run `rustc` to learn about target-specific information

Caused by:
  process didn't exit successfully: `/home/gh-sgasho/.rustup/toolchains/enzyme/bin/rustc - --crate-name ___ --print=file-names -Zautodiff=Enable --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=split-debuginfo --print=crate-name --print=cfg -Wwarnings` (exit status: 1)
  --- stderr
  error: autodiff backend not found in the sysroot: failed to find a `libEnzyme-23` folder in the sysroot candidates:
         * /home/gh-sgasho/rust-worktrees/enzyme-wrapper-cleanup/build/x86_64-unknown-linux-gnu/stage1/lib
    |
    = note: it will be distributed via rustup in the future

After: points out the right candidate dir

RUSTFLAGS="-Zautodiff=Enable" cargo +enzyme run --release
info: `cargo` is unavailable for the active toolchain
info: falling back to "/home/gh-sgasho/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo"
error: failed to run `rustc` to learn about target-specific information

Caused by:
  process didn't exit successfully: `/home/gh-sgasho/.rustup/toolchains/enzyme/bin/rustc - --crate-name ___ --print=file-names -Zautodiff=Enable --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=split-debuginfo --print=crate-name --print=cfg -Wwarnings` (exit status: 1)
  --- stderr
  error: autodiff backend not found in the sysroot: failed to find a `libEnzyme-23.so` in the sysroot candidates:
         * /home/gh-sgasho/rust-worktrees/enzyme-wrapper-cleanup/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib
    |
    = note: it will be distributed via rustup in the future

I also verified that sample autodiff code ran successfully using local build, without breaking anything.

r? @ZuseZ4

[rustbot]

Some changes occurred in compiler/rustc_codegen_llvm/src/llvm/enzyme_ffi.rs

cc @ZuseZ4