Test failure in src/tools/rustdoc-themes when rust.rpath = false

[chrisccoulson]

When rust.rpath is set to false in the build config, the test in src/tools/rustdoc-themes fails like this:

RustdocTheme { compiler: Compiler { stage: 0, host: "x86_64-unknown-linux-gnu" }, target: "x86_64-unknown-linux-gnu" }
> ToolBuild { compiler: Compiler { stage: 0, host: "x86_64-unknown-linux-gnu" }, target: "x86_64-unknown-linux-gnu", tool: "rustdoc-themes", path: "src/tools/rustdoc-themes", mode: Libstd, is_ext_tool: false, extra_features: [] }
c Std { target: "x86_64-unknown-linux-gnu", compiler: Compiler { stage: 0, host: "x86_64-unknown-linux-gnu" } }
c Sysroot { compiler: Compiler { stage: 0, host: "x86_64-unknown-linux-gnu" } }
Building stage0 tool rustdoc-themes (x86_64-unknown-linux-gnu)
running: "/home/chr1s/src/rustc/rust/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "build" "--target" "x86_64-unknown-linux-gnu" "-j" "8" "--release" "--manifest-path" "/home/chr1s/src/rustc/rust/src/tools/rustdoc-themes/Cargo.toml" "--features" "" "--message-format" "json"
Compiling rustdoc-themes v0.1.0 (file:///home/chr1s/src/rustc/rust/src/tools/rustdoc-themes)
Finished release [optimized] target(s) in 4.19 secs
< ToolBuild { compiler: Compiler { stage: 0, host: "x86_64-unknown-linux-gnu" }, target: "x86_64-unknown-linux-gnu", tool: "rustdoc-themes", path: "src/tools/rustdoc-themes", mode: Libstd, is_ext_tool: false, extra_features: [] }
< RustdocTheme { compiler: Compiler { stage: 0, host: "x86_64-unknown-linux-gnu" }, target: "x86_64-unknown-linux-gnu" }
c Assemble { target_compiler: Compiler { stage: 0, host: "x86_64-unknown-linux-gnu" } }
c Libdir { compiler: Compiler { stage: 0, host: "x86_64-unknown-linux-gnu" }, target: "x86_64-unknown-linux-gnu" }
c Sysroot { compiler: Compiler { stage: 2, host: "x86_64-unknown-linux-gnu" } }
c Libdir { compiler: Compiler { stage: 2, host: "x86_64-unknown-linux-gnu" }, target: "x86_64-unknown-linux-gnu" }
c Rustdoc { host: "x86_64-unknown-linux-gnu" }
running: "/home/chr1s/src/rustc/rust/build/x86_64-unknown-linux-gnu/stage0-tools-bin/rustdoc-themes" "/home/chr1s/src/rustc/rust/build/x86_64-unknown-linux-gnu/stage2/bin/rustdoc" "/home/chr1s/src/rustc/rust/src/librustdoc/html/static/themes"
/home/chr1s/src/rustc/rust/build/x86_64-unknown-linux-gnu/stage2/bin/rustdoc: error while loading shared libraries: librustc_driver-b74eb5d75d5b6837.so: cannot open shared object file: No such file or directory

command did not execute successfully: "/home/chr1s/src/rustc/rust/build/x86_64-unknown-linux-gnu/stage0-tools-bin/rustdoc-themes" "/home/chr1s/src/rustc/rust/build/x86_64-unknown-linux-gnu/stage2/bin/rustdoc" "/home/chr1s/src/rustc/rust/src/librustdoc/html/static/themes"
expected success, got: exit code: 1

Traceback (most recent call last):
File "./x.py", line 20, in
bootstrap.main()
File "/home/chr1s/src/rustc/rust/src/bootstrap/bootstrap.py", line 800, in main
bootstrap(help_triggered)
File "/home/chr1s/src/rustc/rust/src/bootstrap/bootstrap.py", line 791, in bootstrap
run(args, env=env, verbose=build.verbose)
File "/home/chr1s/src/rustc/rust/src/bootstrap/bootstrap.py", line 148, in run
raise RuntimeError(err)
RuntimeError: failed to run: /home/chr1s/src/rustc/rust/build/bootstrap/debug/bootstrap test -v src/tools/rustdoc-themes/