run-pass tests in tests/ui-fulldeps/ fall when download-rustc is enabled

[jyn514]

I tried this code: ./configure --set profile=compiler --set rust.download-rustc && x test ui-fulldeps

I expected to see this happen: All tests pass.

Instead, this happened:

failures:
    [ui] tests/ui-fulldeps/deriving-encodable-decodable-cell-refcell.rs
    [ui] tests/ui-fulldeps/compiler-calls.rs
    [ui] tests/ui-fulldeps/issue-14021.rs
    [ui] tests/ui-fulldeps/deriving-encodable-decodable-box.rs
    [ui] tests/ui-fulldeps/stable-mir/crate-info.rs
    [ui] tests/ui-fulldeps/mod_dir_path_canonicalized.rs
    [ui] tests/ui-fulldeps/pprust-expr-roundtrip.rs

test result: FAILED. 63 passed; 7 failed; 1 ignored; 0 measured; 0 filtered out; finished in 3.14s

The failures all look like this:

---- [ui] tests/ui-fulldeps/pprust-expr-roundtrip.rs stdout ----

error: test run failed!
status: exit status: 127
command: cd "/home/jyn/src/rust2/build/x86_64-unknown-linux-gnu/test/ui-fulldeps/pprust-expr-roundtrip" && RUST_TEST_THREADS="16" "/home/jyn/src/rust2/build/x86_64-unknown-linux-gnu/test/ui-fulldeps/pprust-expr-roundtrip/a"
stdout: none
--- stderr -------------------------------
/home/jyn/src/rust2/build/x86_64-unknown-linux-gnu/test/ui-fulldeps/pprust-expr-roundtrip/a: error while loading shared libraries: libLLVM-16-rust-1.70.0-nightly.so: cannot open shared object file: No such file or directory
------------------------------------------

I see the following llvm .so files in the build directory:

; fd libLLVM build/host/ci-rustc*
build/host/ci-rustc/lib/libLLVM-16-rust-1.70.0-nightly.so
build/host/ci-rustc-sysroot/lib/libLLVM-16-rust-1.70.0-nightly.so

but with the rustc-dev component added, it should also be in the rustlib target folder:

; ls $(rustc --print sysroot)/lib/rustlib/x86_64-unknown-linux-gnu/lib/libLLVM*
/home/jyn/.local/lib/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libLLVM-16-rust-1.70.0-nightly.so

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