test(pgo): only run on nightly

So it won't run on rust-lang/rust's CI
because `CARGO_TEST_DISABLE_NIGHTLY` is set on that.

See rust-lang/rust 133675

tests/testsuite/pgo.rs

@@ -22,10 +22,12 @@ fn llvm_profdata() -> Option<PathBuf> {
     })
 }
 
-#[cargo_test]
+// Rustc build may be without profiling support.
+// Mark it as nightly so it won't run on rust-lang/rust CI.
+#[cfg_attr(target_os = "linux", cargo_test(nightly, reason = "rust-lang/rust#133675"))]
 // macOS may emit different LLVM PGO warnings.
 // Windows LLVM has different requirements.
-#[cfg_attr(not(target_os = "linux"), ignore = "linux only")]
+#[cfg_attr(not(target_os = "linux"), cargo_test, ignore = "linux only")]
 fn pgo_works() {
     let Some(llvm_profdata) = llvm_profdata() else {
         return;