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

[Sorixelle]

• Update nixpkgs
• Drop all derivations and related support for the legacy, Python 2-based pebble-tool
• 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
• Various package updates:
  • libpebble2: 0.0.28 -> 0.0.31
  • pebble-tool: 5.0.5 -> 5.0.35
  • pebble-toolchain-bin: 4.5 -> 4.9.169
  • pebble-qemu: 2.5.0-pebble8 -> 10.1.5-pebble14

EDIT: Struck out portions no longer applicable, see comment below. 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.

[Sorixelle]

In the time it's taken us to properly get around to having this ready for merge, the newer GCC stuff has been done by Core Devices. As a result, this PR is now dropping all support for the legacy pebble-tool, and only Core Devices' pebble-tool is supported from now on.