Detect Nix failure due to Git conflict

[ThetaSinner]

There's a code comment explaining this one but I'll post the before and after here to make it clearer. When running Scaffolding inside a Git repository I was getting:

✔ App name (no whitespaces): · asdf
✔ Choose UI framework: · Vue
✔ Do you want to set up the holonix development environment for this project? · Yes (recommended)
Setting up nix development environment...
error: getting status of '/nix/store/2jbblm8bwyv0kq1k9r6dxy7q735d8v2c-source/asdf': No such file or directory
Error: Error setting up the nix environment

After this change, it tells you what the problem is:

✔ App name (no whitespaces): · asdf
✔ Choose UI framework: · Vue
✔ Do you want to set up the holonix development environment for this project? · Yes (recommended)
Error: Error setting up the nix environment - detected that Scaffolding is running inside an existing Git repository, please choose a different location to scaffold

This is very much a Nix flakes gotcha but I think it's helpful to tell people what's happening if possible.