check if using an absolute dir here helps on Windows

Author: RalfJung

tests/testsuite/directory.rs

@@ -805,7 +805,7 @@ fn root_dir_diagnostics() {
     // Crucially, the rustc error message below says `ws_root/...`, i.e.
     // it is relative to our fake home, not to the workspace root.
     p.cargo("check")
-        .arg("-Zroot-dir=.")
+        .arg(format!("-Zroot-dir={}", p.root()))
         .arg("--manifest-path=ws_root/Cargo.toml")
         .masquerade_as_nightly_cargo(&["-Zroot-dir"])
         .with_status(101)