add some kind of cachix-url variable in repos.json

[bignaux]

In order to get the cache name/url easier (for example to display a "cachix use cachename" on nur-search), i think we need some cachix-url, or cachix-name ? I'm pretty beginner on that stuff, so let discuss about that here.

edit :

To make this kind of PR efficient (github-contact etc), i think we need a page mentioned in the nur-packages-templates and other doc, where devel can get notification. in release note would be fine, but would force release for such sort of changes (but why not, since change in repos.json format is a good reason to do it, and watch only release is a good solution since i don't see how to add custom notification).

[Mic92]

This will be possible with flakes later.