feat: nix support (#4012)

very, very, very WIP implementation of nixpkgs for GoReleaser.

**Decisions made for this first version:**
- only linux and darwin, arm64, 386 and amd64
- only support pkgs from goreleaser-generated archives
- no support to push into default nixpkgs repository
- no support to automatically add the _maybe_ new pkg to the root
`default.nix`
- the generated nixpkg will be rather verbose, which shouldn't be too
much of an issue as it is autogenerated anyway

**TODOs**:
- [x] macos universal binary support
- [x] custom pkg path (e.g. pkgs/misc/foo/bar/default.nix)
- [x] handle archives with a folder in them
- [x] add more options: postInstall, ??

**Will be handled in future versions**:
- [ ] archives.format=binary support
- [ ] compile from source
- [ ] PR-ing into nixpkgs
- [ ] armv6l-linux & armv7l-linux support

closes #3537

---------

Signed-off-by: Carlos Alexandro Becker <caarlos0@users.noreply.github.com>

.gitattributes

@@ -9,3 +9,7 @@ www/docs/static/schema.json linguist-generated=true
 www/docs/static/schema-pro.json linguist-generated=true
 www/docs/static/releases.json linguist-generated=true
 www/docs/static/releases-pro.json linguist-generated=true
+*.nix.golden linguist-language=Nix
+*.rb.golden linguist-language=Ruby
+*.json.golden linguist-language=JSON
+*.yaml.golden linguist-language=YAML