Investigate using chezmoi templating and deprecating the keyring transform

[VorpalBlade]

Check how viable this is, it would reduce binary size, avoid the sizable dependency (on *nix) on zbus and associated compile times.

The support can already be compiled out by disabling the keyring cargo feature (on by default).

[VorpalBlade]

Likely not viable, as it won't work over ssh (where it currently works and we just keep whatever the existing value in the config file is).

[VorpalBlade]

This turned out not viable, as chezmoi error handling makes everything abort. For my usecase of sometimes applying config updates to every computer at once over SSH, that behaviour doesn't work.