Config migration causes problems on NixOS

The migration feature causes lazygit to not open anymore on NixOS, as the config file in on a read-only file system. It would be great, if this would only throw a warning instead of not starting at all.
Here is an example of a failed migration:

```
The user config file /home/USER/.config/lazygit/config.yml must be migrated. Attempting to do this automatically.
The following changes were made:

- Changed 'subprocess: true' to 'output: terminal' in custom command

2025/05/26 04:18:21 While attempting to write back migrated user config to /home/USER/.config/lazygit/config.yml, an error occurred: open /home/USER/.config/lazygit/config.yml: read-only file system
```