fix(nix): package build errors

[TechnoStrife]

Summary

• added pygments to nix package dependencies
• downgraded pygments required version to 2.20 because 2.21 is not yet available in nixpkgs

Testing

Current version does not build:

$ nix build github:TagStudioDev/TagStudio#packages.x86_64-linux.tagstudio
error: Cannot build '/nix/store/hqmkkwsi1zfx903nl7kb4nx540ivakc8-TagStudio-9.6.4.drv'.
       Reason: builder failed with exit code 1.
       Output paths:
         /nix/store/8g117lr5jshwi0myibrc45ggkwqjr9bp-TagStudio-9.6.4
         /nix/store/n7yps9dm1gpccdaq54lxzixwb3qfg9pd-TagStudio-9.6.4-dist
       Last 25 log lines:
       > [...]
       > Checking runtime dependencies for tagstudio-9.6.4-py3-none-any.whl
       >   - pygments not installed
       For full logs, run:
         nix log /nix/store/hqmkkwsi1zfx903nl7kb4nx540ivakc8-TagStudio-9.6.4.drv

After the fix, build succeeds:

$ nix build github:TechnoStrife/TagStudio#packages.x86_64-linux.tagstudio && echo 'ok'
ok

Launching the app, it appears to work correctly.

• Platforms Tested:
  • Windows x86
  • Windows ARM
  • macOS x86
  • macOS ARM
  • Linux x86
  • Linux ARM
• Tested For:
  • Basic functionality
  • PyInstaller executable
  • nix package builds

[CyanVoxel]

Thank you for the fix!