Fix x test --stage 1 ui-fulldeps on macOS (until the next beta bump)

"stage 1" for fulldeps means "compile with stage 0, link against stage 1".
But this code wanted to switch on the compiler that's building, not the
compiler that's being tested. Fix the check.

Previously, it would fail with a warning about linker-messages:
```
--- stderr -------------------------------
warning[E0602]: unknown lint: `linker_messages`
   |
   = note: requested on the command line with `-A linker_messages`
   = note: `#[warn(unknown_lints)]` on by default
```

Author: jyn514

src/bootstrap/src/core/build_steps/test.rs

@@ -1840,19 +1840,21 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
             }
         }
 
+        // FIXME(136096): on macOS, we get linker warnings about duplicate `-lm` flags.
+        // NOTE: `stage > 1` here because `test --stage 1 ui-fulldeps` is a hack that compiles
+        // with stage 0, but links the tests against stage 1.
+        // cfg(bootstrap) - remove only the `stage > 1` check, leave everything else.
+        if suite == "ui-fulldeps" && compiler.stage > 1 && target.ends_with("darwin") {
+            flags.push("-Alinker_messages".into());
+        }
+
         let mut hostflags = flags.clone();
         hostflags.push(format!("-Lnative={}", builder.test_helpers_out(compiler.host).display()));
         hostflags.extend(linker_flags(builder, compiler.host, LldThreads::No));
 
         let mut targetflags = flags;
         targetflags.push(format!("-Lnative={}", builder.test_helpers_out(target).display()));
 
-        // FIXME: on macOS, we get linker warnings about duplicate `-lm` flags. We should investigate why this happens.
-        if suite == "ui-fulldeps" && target.ends_with("darwin") {
-            hostflags.push("-Alinker_messages".into());
-            targetflags.push("-Alinker_messages".into());
-        }
-
         for flag in hostflags {
             cmd.arg("--host-rustcflags").arg(flag);
         }