Remove the now-unneeded llvm-tools-preview

Since a working `nm` is no longer needed as part of CI, the rustup
component can be removed.

library/compiler-builtins/.github/workflows/main.yaml

@@ -119,7 +119,6 @@ jobs:
         rustup update "$channel" --no-self-update
         rustup default "$channel"
         rustup target add "${{ matrix.target }}"
-        rustup component add llvm-tools-preview
     - uses: taiki-e/install-action@nextest
     - uses: Swatinem/rust-cache@v2
       with:

library/compiler-builtins/crates/symbol-check/src/main.rs

@@ -84,7 +84,7 @@ fn exec_cargo_with_args(args: &[&str]) -> Vec<PathBuf> {
         }
     }
 
-    cmd.wait().expect("failed to wait on Cargo");
+    assert!(cmd.wait().expect("failed to wait on Cargo").success());
 
     assert!(!check_files.is_empty(), "no compiler_builtins rlibs found");
     println!("Collected the following rlibs to check: {check_files:#?}");