Use CMake imported targets and a system argparse instead of FetchContent.

The upstream link command `target_link_libraries(... ${NVML_LIB_NAME} cuda)`
expands to plain `-lnvidia-ml -lcuda`, which forces the linker to look up
those names on its search path. The corresponding stubs live in non-standard
`*/lib/stubs` directories that aren't on the Nix linker search path, so the
build fails. Switching to the `CUDA::nvml` / `CUDA::cuda_driver` imported
targets exposed by `find_package(CUDAToolkit)` makes CMake link against the
absolute stub paths it discovers via `CUDAToolkit_ROOT` instead.

The `FetchContent` argparse dependency is also dropped in favour of the
`argparse` package from Nixpkgs (pinned to the same v3.2 upstream requests):
`FetchContent` downloads a tarball over the network, which isn't available in
the Nix sandbox.
---
 CMakeLists.txt | 12 +++---------
 1 file changed, 3 insertions(+), 9 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -76,14 +76,8 @@
     set(CMAKE_BUILD_TYPE "Release")
 endif()

-include(FetchContent)
-FetchContent_Declare(
-        argparse
-        URL https://github.com/p-ranav/argparse/archive/refs/tags/v3.2.tar.gz
-        URL_HASH SHA256=9dcb3d8ce0a41b2a48ac8baa54b51a9f1b6a2c52dd374e28cc713bab0568ec98
-        # Pinned version - provides .flag(), .store_into(), .choices()
-)
-FetchContent_MakeAvailable(argparse)
+find_package(argparse REQUIRED)
+find_package(CUDAToolkit REQUIRED)

 set(src
     cuda_version_check.cpp
@@ -107,7 +101,7 @@
 target_include_directories(nvbandwidth PRIVATE
     ${CMAKE_CURRENT_BINARY_DIR}
     ${CMAKE_CUDA_TOOLKIT_INCLUDE_DIRECTORIES}
     .)
-target_link_libraries(nvbandwidth argparse ${NVML_LIB_NAME} cuda)
+target_link_libraries(nvbandwidth argparse::argparse CUDA::nvml CUDA::cuda_driver)
 if(NOT WIN32)
     target_link_libraries(nvbandwidth dl)
 endif()