fix: require root module to configure nixpkgs

[avrabe]

Summary

Fix module extension isolation error: nix_repo.github() can only be used in the root module.

Problem

When rules_rocq_rust is used as a dependency, the nix_repo.github() call in its MODULE.bazel fails with:

Illegal use of the github tag. The github tag may only be used on an isolated module extension or in the root module

Solution

Remove the nixpkgs configuration from rules_rocq_rust's MODULE.bazel. Users must configure it in their root MODULE.bazel:

nix_repo = use_extension("@rules_nixpkgs_core//extensions:repository.bzl", "nix_repo")
nix_repo.github(
    name = "nixpkgs",
    org = "NixOS",
    repo = "nixpkgs",
    commit = "d4f247e89f6e10120f911e2e2d2254a050d0f732",
    sha256 = "dee2f93be38d1a63ee9b731fa993f0ac2c60c3a62cca32d4cc64c2ed5b8a8e06",
)
use_repo(nix_repo, "nixpkgs")

Test plan

• CI passes
• Test in LOOM with nixpkgs configured in root module