nix: ability to use dependencies from different nixpkgs versions

[rdnt]

I think it would be valuable to have the ability to specify nixpkgs version on a per-task basis. Would be useful for cases like having the latest version of a specific linter, but also using stable versions for everything else.

Not sure if it is feasible, but maybe worth investigating.

[zuzuleinen]

I think it's a good idea. We can even make pinning nixpkgs per-package level.

We should also check rules_nixpkgs for inspiration:

nixpkgs_git_repository(
    name = "nixpkgs",
    revision = "17.09", # Any tag or commit hash
    sha256 = "" # optional sha to verify package integrity!
)

nixpkgs_package(
    name = "hello",
    repositories = { "nixpkgs": "@nixpkgs//:default.nix" }
)