test: add -Zunstable-options with custom targets

rustc is destabilising custom targets (rust-lang/rust#150151) and
requiring that `-Zunstable-options` is passed when custom targets are
used. This is the minimum change required so that the rustc patch can be
merged.

Author: davidtwco

tests/testsuite/rustc.rs

@@ -840,6 +840,7 @@ RUST_TARGET_PATH = { value = "./targets", relative = true }
 
     p.cargo("rustc -Z unstable-options --print cfg")
         .masquerade_as_nightly_cargo(&["print"])
+        .env("RUSTFLAGS", "-Z unstable-options")
         .with_stdout_data(str!["..."].unordered())
         .run();
 }