[hooks_runner] Pass NIX_ environment variables through in hooks_runner

[GregoryConrad]

Allows environment variables starting with NIX_ to pass through in hooks_runner. This fixes failing builds when using Nix-installed LLVM.

Fixes #2623

[GregoryConrad]

CC @dcharkes

Update: I was able to verify this PR fixes my issue in flutter_tools, but still no idea how to update my local dart's copy of hooks_runner. Not super important since I verified this fixes my problem via Flutter, but would be curious as to how that works with the dart tool.

Also, what's the mechanism to test this change locally (against my actual project)? I tried adding the following to my pubspec.yaml:

dependency_overrides:
  hooks_runner:
    path: /path/to/native/pkgs/hooks_runner

But no dice, my version wasn't used. Doesn't seem like hooks_runner is in the pubspec.lock without that override either, indicating it's injected some other way. (edit: yea, contributing guide says it "rolls" into Dart/Flutter. How can I "roll" it into my local copy of Dart/Flutter? edit 2: was able to get my local flutter to point to it via manually editing flutter_tools/pubspec.*, but still no idea how to try this on the dart tool side)