fix: Add NixOS environment variables to default passthroughs (#12417)
## Summary
- Adds `__NIXOS_*` and `NIX_*` to `BUILTIN_PASS_THROUGH_ENV`
On NixOS, when `__NIXOS_SET_ENVIRONMENT_DONE` is stripped from the
environment, spawning a non-interactive shell resets `PATH` via
`/etc/zshenv` (or equivalent). This silently breaks task execution since
the shell loses most of its `PATH` entries.
These variables have no effect on non-NixOS systems.
Closes #11688