Make symlink path configurable

[jdonszelmann]

There used to be a comment here https://github.com/Mic92/sops-nix/blob/cfdbaf68d00bc2f9e071f17ae77be4b27ff72fa6/modules/sops/default.nix#L121

asking whether this needed to be configurable? Well, I needed it to be configurable. So I made it so. I thought I'd contribute the change upstream. What do you think?

I'm sorry, I alro ran autoformat on the file. Maybe that's nice to do on the entire project, though I can undo it if you really want to.

Oh, and feel free to squash. The separate commits don't make that much sense.