test: add -Zunstable-options with custom targets (#16467)

rustc is destabilising custom targets (rust-lang/rust#150151) by
requiring that `-Zunstable-options` is passed when custom targets are
used. This patch is the minimum change required so that the rustc patch
can be merged without failing the Cargo tests run in rustc's CI.

Author: weihanglo

tests/testsuite/rustc.rs

@@ -821,7 +821,7 @@ windows
         .run();
 }
 
-#[cargo_test]
+#[cargo_test(nightly, reason = "custom targets are unstable in rustc")]
 fn rustc_with_print_cfg_config_toml_env() {
     let p = project()
         .file("Cargo.toml", &basic_bin_manifest("foo"))
@@ -841,6 +841,7 @@ RUST_TARGET_PATH = { value = "./targets", relative = true }
     p.cargo("rustc -Z unstable-options --print cfg")
         .masquerade_as_nightly_cargo(&["print"])
         .with_stdout_data(str!["..."].unordered())
+        .env("RUSTFLAGS", "-Z unstable-options")
         .run();
 }