Quickstart can't build hello world: librustc_codegen_gcc.so not found

I'm trying to build librustc_codegen_gcc.so from this repo (rather than from Rust) to make it easier to upstream patches. However, the Quickstart instructions do not appear to work.

Following the instructions, I've:

1. Copied config.example.toml to config.toml
2. Ran ./y.sh prepare
3. Ran ./y.sh build --sysroot --release

Now I'm trying to run the fourth step:

$ ./y.sh cargo build --manifest-path tests/hello-world/Cargo.toml
[BUILD] build system
    Finished [`release` profile [optimized]](https://doc.rust-lang.org/cargo/reference/profiles.html#default-profiles) target(s) in 0.01s
Using `/home/seancross/Code/rustc_codegen_gcc/build/libgccjit/b794cb99508e7eccd40fa42bb310d5781e09899b` as path for libgccjit
   Compiling hello_world v0.0.0 (/home/seancross/Code/rustc_codegen_gcc/tests/hello-world)
error: couldn't load codegen backend /home/seancross/Code/rustc_codegen_gcc/target/debug/librustc_codegen_gcc.so: cannot open shared object file: No such file or directory

error: could not compile `hello_world` (bin "hello_world") due to 1 previous error
$

It appears to be looking for something under target/debug even though the instructions say to run ./y.sh build --sysroot --release. If I re-run step 3 with ./y.sh build --sysroot without --release, then attempting to build hello-world gives a different error:

$ ./y.sh cargo build --manifest-path tests/hello-world/Cargo.toml
[BUILD] build system
    Finished [`release` profile [optimized]](https://doc.rust-lang.org/cargo/reference/profiles.html#default-profiles) target(s) in 0.00s
Using `/home/seancross/Code/rustc_codegen_gcc/build/libgccjit/b794cb99508e7eccd40fa42bb310d5781e09899b` as path for libgccjit
   Compiling hello_world v0.0.0 (/home/seancross/Code/rustc_codegen_gcc/tests/hello-world)
error[E0463]: can't find crate for `mylib`
 --> src/main.rs:1:5
  |
1 | use mylib::my_func;
  |     ^^^^^ can't find crate

For more information about this error, try `rustc --explain E0463`.
error: could not compile `hello_world` (bin "hello_world") due to 1 previous error
$

Start with the Quickstart instructions, config.example.toml, and the ./y.sh prepare/build/cargo entry points; reproduce the release and non-release commands against tests/hello-world/Cargo.toml. Trace why the release build looks under target/debug and why the non-release build cannot find mylib. Done means the documented Quickstart commands build hello-world successfully.