bootstrap fails to find `codegen-backends` folder

[onur-ozkan]

When download-rustc is enabled, using multiple codegen backends in build configuration (rust.codegen-backends= ["llvm", "gcc"]) leads bootstrap to fail with the following error:

$ x --stage 2 test
...
...
Testing GCC stage2 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
    Finished `release` profile [optimized] target(s) in 0.00s
     Running `/home/nimda/devspace/.other/rustc-builds/build/x86_64-unknown-linux-gnu/stage2-codegen/x86_64-unknown-linux-gnu/release/y test --use-system-gcc --use-backend gcc --out-dir /home/nimda/d
evspace/.other/rustc-builds/build/x86_64-unknown-linux-gnu/stage2-tools/cg_gcc --release --mini-tests --std-tests`
Using system GCC
[BUILD] mini_core
error: failed to find a `codegen-backends` folder in the sysroot candidates:
       * /home/nimda/devspace/.other/rustc-builds/build/x86_64-unknown-linux-gnu/stage2
       * /home/nimda/devspace/.other/rustc-builds/build/x86_64-unknown-linux-gnu/stage2