fix more broken URLs

[fricklerhandwerk]

Tested with netlify-cli!

Note that this changes semantics, and now redirects unstable to the latest Nix version rather than to the most recent build from master. This is to avoid

• either more downstream errors with "pretty URLs", which are not supported by Hydra
• or expensively re-building the manual on nix.dev regularly just to work around Hydra's limitations

This is probably what we want. I assume that most people will use the latest Nix release rather than master. For those who live on master there's still https://nix.dev/manual/nix/development, and presumably being power users they will hopefully know that.

Closes:

• https://github.com/NixOS/nix.dev/issues/1005
• https://github.com/NixOS/nix.dev/issues/1006
• https://github.com/NixOS/nixos-homepage/issues/1483
• https://github.com/NixOS/nix.dev/issues/996
• https://discourse.nixos.org/t/avoid-string-interpolation-nix/30772/9

Thanks everyone for participating in this involuntary scream test, continuing our year-long tradition of compaint-based, anger-driven development. ;)

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/avoid-string-interpolation-nix/30772/14