Update nixpkgs to version 25.05 in flake.nix

[Jakio815]

The updated pico-sdk version (2.2.0) required picotool (2.1.1), and the nix package manager was not fetching the new picotool version. This ended up to download the new picotool everytime we are doing lfc.

CMake Warning at /Users/dkim314/project/temp/pico-sdk/tools/Findpicotool.cmake:30 (message):
  No installed picotool with version 2.1.1 found - building from source

  It is recommended to build and install picotool separately, or to set
  PICOTOOL_FETCH_FROM_GIT_PATH to a common directory for all your SDK
  projects
Call Stack (most recent call first):
  /Users/dkim314/project/temp/pico-sdk/tools/CMakeLists.txt:168 (find_package)
  /Users/dkim314/project/temp/pico-sdk/tools/CMakeLists.txt:688 (pico_init_picotool)
  /Users/dkim314/project/temp/pico-sdk/src/cmake/on_device.cmake:81 (picotool_postprocess_binary)
  CMakeLists.txt:66 (pico_add_extra_outputs)


Downloading Picotool

This PR updates nixpkgs to version 25.05 in flake.nix, and will fetch the new picotool version. Thanks to @the-systematic-chaos

[Jakio815]

@hokeun Looks like I can't request reviews. Could you please have a look?