Have ./mach cargo not change to components/servo.

`ports/android/glut` relies on it executing on the current directory.

python/servo/devenv_commands.py

@@ -22,7 +22,7 @@ class MachCommands(CommandBase):
         help="Command-line arguments to be passed through to Cargo")
     def cargo(self, params):
         return subprocess.call(["cargo"] + params,
-                               env=self.build_env(), cwd=self.servo_crate())
+                               env=self.build_env())
 
     @Command('update-cargo',
              description='Update Cargo dependencies',