Fix cargo-gpu in build script failing when called by Miri or Clippy

[tuguzT]

cargo-gpu Rust-GPU/cargo-gpu#101

@Firestar99: I've added a new CargoCmd struct that should handle all env vars in one central place for both spirv-builder and cargo-gpu install. In case of a failure, it also debug! logs the env vars which were removed and which were inherited with their values, to make debugging these sort of issues easier in the future.

As stated in Rust-GPU/cargo-gpu#93, build script which uses rustc_codegen_spirv via cargo-gpu failed to build shader crate while using Miri.
This pull request fixes this issue by removing RUSTC_WRAPPER environment variable from cargo call.

[tuguzT]

Please look at Rust-GPU/cargo-gpu#101 for additional context