Set a configuration value via environment variable

### Is your feature request related to a problem? Please describe.
I'd like to have a modified configuration in specific contexts. Currently to do so, unless I'm missing something, I need to maintain a second configuration file. For instance, I generally use `os.editPreset: 'nvim'`, but in the context of Zed, I want to use `zed`.

### Describe the solution you'd like
I'd like to be able to set `LAZYGIT_OS_PRESET=zed` to override just this specific value while still using the rest of the config.yml.

### Describe alternatives you've considered
I can maintain a second config and use it explicitly, but then for other changes in configuration, I must update both files.

### Additional context

FWIW, I can work around this specific issue by removing the value altogether, because `EDITOR` and `VISUAL` are correctly utilized. But I still think overrides is a worthy consideration.