nix shell: add required pyserial

[evils]

Description

#16584 added pyserial as a required dependency, but the nix environment does not provide it, so i can't build firmware
this adds pyserial to the nix environment to fix that

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

i'm not sure if there's a dedicated maintainer of the nix stuff to ping...
but thanks for providing a nix environment :)

[sigprof]

I actually prepared a much more extensive update in #18131 (it also updates QMK CLI to 1.1.1, which has the qmk-dotty-dict → dotty-dict change, which then requires further updates and Darwin-specific workarounds; all other Python packages also get updated to their current releases to match the typical result of QMK setup on other systems).

[evils]

closing in favour of #18131