python3Packages.setuptools-rust: fix passthru test

The build wanted to create a Cargo.lock in the top-level source
directory (presumably because it is also a Cargo project), since we
were crating the lock file in sourceRoot (which was set to the example
project). This failed because the sources are read-only. Change the
test to create the lock file in the top-level crate directory, which
is Cargo's default behavior.

pkgs/development/python-modules/setuptools-rust/pyo3-test/default.nix

@@ -17,5 +17,9 @@ callPackage ../../../tools/rust/maturin/pyo3-test/generic.nix {
     rust.rustc
   ]);
 
-  sourceRoot = "source/examples/word-count";
+  preConfigure = ''
+    # sourceRoot puts Cargo.lock in the wrong place due to the
+    # example setup.
+    cd examples/word-count
+  '';
 }