Bump nixpkgs, make Core Devices' pebble-tool the default in development shells

[Sorixelle]

• Update nixpkgs
• Use the Lix project's flake-compat in place of Eelco's
• Drop the dependency on flake-utils
• Split the overlay in the flake into it's own dedicated file, for easier consumption by non-flake users
• Define pebbleEnv with mkShellNoCC to avoid issues where pebble-tool tries to build Pebble apps with the stdenv's toolchain
• Mark arm-embedded-toolchain as broken - seems to no longer compile with latest nixpkgs
• Mark pebble-tool as broken - nixpkgs are removing Python 2 support ([Tracking issue] Removing python2 leftovers NixOS/nixpkgs#479927)
• Various package updates:
  • libpebble2: 0.0.28 -> 0.0.30
  • coredevices.pebble-tool: 5.0.5 -> 5.0.21
  • pebble-toolchain-bin: 4.5 -> 4.9.77

Once this is merged, I'm going to be calling support for the original, Python 2-based pebble-tool dead. In a world with Core Devices' pebble-tool, it's largely unnecessary, and continuing to maintain it here is proving to be an exercise in futility, especially now with nixpkgs moving to kill Python 2 completely.

I'm keeping most of the relevant code around for now, in case I get around to fixing it back up again. Once Core Devices implements support for newer GCC toolchains in their SDK, I'll probably remove it all, since trying to support multiple toolchain versions in here doesn't sound like a fun time. In the event I don't get around to fixing it, consider 54d0893 to be the last commit with working support for the old pebble-tool.