build script: `cargo clippy` fails

[Firestar99]

Requires #71

Running plain cargo clippy in a project using cargo-gpu as a dependency from a build script fails. There's likely some env of clippy seeping into our build script, making the inner cargo launched by spirv-builder fail.

error: failed to run custom build command for `colorbubble v0.1.0 (/home/firestar99/workspace/colorbubble)`

Caused by:
  process didn't exit successfully: `/home/firestar99/workspace/colorbubble/target/debug/build/colorbubble-e97d294aaa7f8781/build-script-build` (exit status: 1)
  --- stderr
  error: failed to run `rustc` to learn about target-specific information

  Caused by:
    process didn't exit successfully: `/home/firestar99/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/bin/clippy-driver /home/firestar99/.rustup/toolchains/nightly-2025-05-09-x86_64-unknown-linux-gnu/bin/rustc - --crate-name ___ --print=file-names -Zcodegen-backend=/home/firestar99/.cache/rust-gpu/codegen/https___github_com_rust-gpu_rust-gpu+c052ee5b/librustc_codegen_spirv.so -Zbinary-dep-depinfo -Csymbol-mangling-version=v0 '-Zcrate-attr=feature(register_tool)' '-Zcrate-attr=register_tool(rust_gpu)' -Coverflow-checks=off -Cdebug-assertions=off -Zinline-mir=off -Zmir-enable-passes=-GVN --target /home/firestar99/.cache/rust-gpu/codegen/https___github_com_rust-gpu_rust-gpu+c052ee5b/target-specs/spirv-unknown-vulkan1.2.json --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=sysroot --print=split-debuginfo --print=crate-name --print=cfg -Wwarnings` (exit status: 1)
    --- stderr
    error: the option `Z` is only accepted on the nightly compiler

    help: consider switching to a nightly toolchain: `rustup default nightly`

    note: selecting a toolchain with `+toolchain` arguments require a rustup proxy; see <https://rust-lang.github.io/rustup/concepts/index.html>

    note: for more information about Rust's stability policy, see <https://doc.rust-lang.org/book/appendix-07-nightly-rust.html#unstable-features>

    error: 1 nightly option were parsed

  Error: BuildFailed