flake.lock: Update (#839)

Flake lock file updates:

• Updated input 'neovim-nightly':
    'github:nix-community/neovim-nightly-overlay/0f601090d4d54b3da0d03e270cb6a5c68bf84dd3?narHash=sha256-uP5SbbbN86%2BLZ8VubL01UKD6bez5DK9prqIqQOMy3Jw%3D' (2026-02-21)
  → 'github:nix-community/neovim-nightly-overlay/9ea828da7d7a4f21beb909a541a2da790031059c?narHash=sha256-3l%2BXqvGhtEAgO/Bmj0BztZzt/B8/TaeXAueJLZnGWXg%3D' (2026-02-22)
• Updated input 'neovim-nightly/neovim-src':
    'github:neovim/neovim/d79a9dcd422133bc1e4b4ef94444962560d7a6d7?narHash=sha256-7RPG%2BRG/e0O79HjNT/ztC7K7j/xXazltq3TPk1mauqY%3D' (2026-02-20)
  → 'github:neovim/neovim/940d5c239b088fe0e994a220b4d524ddb44760e9?narHash=sha256-sH9AWegmJXG6cDCW7FtbTAcdno8KODvh2FwHozeyDgk%3D' (2026-02-21)

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: mrcjkb