setup: fix a possible NameError on rust build

  File "setup.py", line 975, in rustbuild
    "command: %r, environment: %r" % (self.rustsrcdir, cmd, env))
NameError: global name 'cmd' is not defined

setup.py

@@ -1090,7 +1090,7 b' class RustExtension(Extension):'
         except subprocess.CalledProcessError:
             raise RustCompilationError(
                 "Cargo failed. Working directory: %r, "
-                "command: %r, environment: %r" % (self.rustsrcdir, cmd, env))
+                "command: %r, environment: %r" % (self.rustsrcdir, cargocmd, env))
 
 class RustEnhancedExtension(RustExtension):
     """A C Extension, conditionally enhanced with Rust code.