skip enzyme only in dry run, error when failing otherwise. addressing Onur's feedback

Author: ZuseZ4

src/bootstrap/src/core/build_steps/compile.rs

@@ -1178,7 +1178,8 @@ pub fn rustc_cargo(
         let enzyme_dir = builder.build.out.join(arch).join("enzyme").join("lib");
         cargo.rustflag("-L").rustflag(enzyme_dir.to_str().expect("Invalid path"));
 
-        if let Some(llvm_config) = builder.llvm_config(builder.config.build) {
+        if !builder.config.dry_run() {
+            let llvm_config = builder.llvm_config(builder.config.build).unwrap();
             let llvm_version_major = llvm::get_llvm_version_major(builder, &llvm_config);
             cargo.rustflag("-l").rustflag(&format!("Enzyme-{llvm_version_major}"));
         }