fix envvar CARGO_CFG_TARGET_ARCH set at runtime vs compile time

backends/trtllm/build.rs

@@ -128,7 +128,7 @@ fn main() {
     // NCCL is slightly trickier because it might not have a pkgconfig installed
     let nccl_library_path = NCCL_ROOT_DIR.unwrap_or(&format!(
         "/usr/local/{}-linux-gnu",
-        env!("CARGO_CFG_TARGET_ARCH")
+        env::var("CARGO_CFG_TARGET_ARCH").unwrap()
     ));
     println!(r"cargo:rustc-link-search=native={}", nccl_library_path);
     println!("cargo:rustc-link-lib=dylib=nccl");