$CARGO_MANIFEST_DIR "leakage" (used at runtime) due to target-specs vendoring.

I cant keep rust-gpu updated for my projects since this commit

Example & Steps To Reproduce

I have made a minimal example to demonstrate

nix run github:abel465/rust-gpu-nixos-example/b117267d254310df77e69debde9e510ed193ff56

The shader builds just fine for the above command. it uses this version of rust-gpu

> nix run github:abel465/rust-gpu-nixos-example/6c9e2ca63da09ab1d119a06a596cd41f84255876
cargo:rerun-if-env-changed=RUSTGPU_CODEGEN_ARGS
cargo:rerun-if-env-changed=RUSTGPU_RUSTFLAGS
error: target path "/build/cargo-vendor-dir/spirv-builder-0.9.0/target-specs/spirv-unknown-vulkan1.1.json" is not a valid file

Caused by:
  No such file or directory (os error 2)
thread 'main' panicked at runner/src/main.rs:17:42:
called `Result::unwrap()` on an `Err` value: BuildFailed
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

The above command is the same except it uses the next commit of rust-gpu which seems to hard code CARGO_MANIFEST_DIR into the binary. This is problematic because the build environment is no longer available during runtime with nix

System Info

• Rust: 1.78.0-nightly (3b1717c05 2024-03-10)

Start by comparing the behavior introduced in commit 775b5d5 with the working version 40456ae, focusing on runner/src/main.rs and the target-specs path shown in the reproduction. Run the two nix commands from the issue; done means the vendored shader builds without relying on the unavailable build-environment path.