remove RUSTC env var to prevent build script toolchain leaking into cargo gpu

This is bandaid fix, it now ignores RUSTC environment variable set from elsewhere, like when shader crate is part of a workspace. As of today, it fails to compile on workspace which uses nightly because an additional restriction was added to target specification json file on nightly. Even though enforcing specific rustc version seems like good idea for the purpose of cargo-gpu, target specification json file should be updated as well if we were to ever bump up required rustc version.

Author: tmvkrpxl0

crates/cargo-gpu/src/install.rs

@@ -202,7 +202,8 @@ package = "rustc_codegen_spirv"
             build_command
                 .current_dir(&checkout)
                 .arg(format!("+{}", spirv_version.channel))
-                .args(["build", "--release"]);
+                .args(["build", "--release"])
+                .env_remove("RUSTC");
 
             log::debug!("building artifacts with `{build_command:?}`");