fix: force nng checkout

to prevent build failures like

error: The following untracked working tree files would be overwritten by checkout:
        cmake/FindmbedTLS.cmake

seems to be caused by case sensitivity differences beween file systems (FindmbedTLS.cmake vs FindMbedTLS.cmake)

setup.py

@@ -63,7 +63,7 @@ def run(self):
         """Clone nng and build it with cmake, with TLS enabled."""
         if not os.path.exists(self.git_dir):
             check_call("git clone {}".format(self.repo), shell=True)
-            check_call("git checkout {}".format(self.rev), shell=True, cwd=self.git_dir)
+            check_call("git checkout -f {}".format(self.rev), shell=True, cwd=self.git_dir)
         if not os.path.exists(self.build_dir):
             os.mkdir(self.build_dir)