Firmware stops building without changing anything in the git repo

[MangoIV]

• without any changes the firmware just stops building
• this is not desirable, my expectation would be that without any changes the build works reproducible and creates the same outputs
• see https://github.com/MangoIV/le_chiff_ble_config for reference
• I propose using nix flakes for locking the build inputs to a revision such that we won't have firmware breaking over time
• relevant https://github.com/moergo-sc/zmk/blob/main/README-NIX.md (this probably needs an upgrade to support flakes, so that we can lock the build inputs)
• I maybe can take this three weeks from now (at least the flakification)

[petejohanson]

If you really don't want the version of ZMK changing in your keymap, you an pin a specific ZMK sha1 in the west.yml of your user config repo, and it will always build against that revision until you upgrade.

No need to move to NIX or upend things.