Use the system-wide installed nanobind instead of vendoring it via Git submodules.

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 92eab9d..3bc8b0d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -44,8 +44,11 @@ add_definitions(
   -DSWIFT_STDLIB_HAS_TYPE_PRINTING=1
 )
 
-# finding nanobind
-add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/src/nanobind)
+# See https://nanobind.readthedocs.io/en/latest/building.html#finding-nanobind
+execute_process(
+  COMMAND "${Python_EXECUTABLE}" -m nanobind --cmake_dir
+  OUTPUT_STRIP_TRAILING_WHITESPACE OUTPUT_VARIABLE nanobind_ROOT)
+find_package(nanobind CONFIG REQUIRED)
 
 nanobind_add_module(
 	pydemumble