nix: add paramiko to boards environment for flashing over SSH

[charlesbaynham]

ARTIQ Pull Request

Description of Changes

Adds paramiko to the lightweight boards nix environment so that it can be used to flash using remote targets (e.g. raspberry pis).

Type of Changes

	Type
✓	:sparkles: New feature

Steps (Choose relevant, delete irrelevant before submitting)

All Pull Requests

• [x] Use correct spelling and grammar.
• [ ] Update RELEASE_NOTES.rst if there are noteworthy changes, especially if there are changes to existing APIs.
• [ ] Close/update issues.
• [x] Check the copyright situation of your changes and sign off your patches (git commit --signoff, see copyright).

Code Changes

• [ ] Run flake8 to check code style (follow PEP-8 style). flake8 has issues with parsing Migen/gateware code, ignore as necessary.
• [x] Test your changes or have someone test them. Mention what was tested and how.
• [ ] Add and check docstrings and comments
• [ ] Check, test, and update the unittests in /artiq/test/ or gateware simulations in /artiq/gateware/test

Documentation Changes

• [ ] Check, test, and update the documentation in doc/. Build documentation (nix build .#artiq-manual-html; nix build .#artiq-manual-pdf) to ensure no errors.

Git Logistics

• [x] Split your contribution into logically separate changes (git rebase --interactive). Merge/squash/fixup commits that just fix or amend previous commits. Remove unintended changes & cleanup. See tutorial.

• [x] Write short & meaningful commit messages. Review each commit for messages (git show). Format:

  topic: description. < 50 characters total.
  
  Longer description. < 70 characters per line

Licensing

See copyright & licensing for more info. ARTIQ files that do not contain a license header are copyrighted by M-Labs Limited and are licensed under LGPLv3+.