chore: Fix TRIPLE env value used in external tests (#2082)

When trying cross tests for powerpc64le, I noticed that a mold test
marked for skipping would fail with an error indicating that
`ppc64le-linux-gnu-gcc` could not be found. This occurs because the
`TRIPLE` environment variable is being constructed using
`format!("{}-linux-gnu", arch)`, which incorrectly sets the GCC command.

Author: lapla-cogito

wild/tests/external_tests/mold_tests.rs

@@ -44,7 +44,7 @@ fn run_mold_test(mold_test: &Path) -> Result<Output> {
         && let Ok(arch) = Architecture::from_str(arch_name)
         && arch != get_host_architecture()
     {
-        Some(format!("{}-linux-gnu", arch))
+        Some(arch.cross_triplet())
     } else {
         None
     };