fix(rust): resolve architectural inconsistency in nixpkgs handling

## Problem

The rust kernel had an architectural inconsistency where it used two different
approaches to access nixpkgs packages:

1. **Module-level**: `requiredRuntimePackages = [config.nixpkgs.cargo ...]`
2. **Function-level**: `pkgs = import config.nixpkgs.path { ... }`

This caused evaluation failures when `config.nixpkgs.cargo` was undefined,
since users were only providing nixpkgs configuration (path + overlays)
rather than an instantiated package set.

## Root Cause Analysis

**Timeline of the inconsistency:**

- **Feb 9, 2023** (commit 9fc7d31): GuangTao Zhang introduced centralized
  `config.nixpkgs` system. Rust kernel correctly used `config.nixpkgs.*`

- **Mar 16, 2023** (commit 0fea293): Dan Baker's "refactor/turn kernels into
  modules" **accidentally combined two patterns**:
  - Preserved module-level `config.nixpkgs.cargo` (correct for that system)
  - Added function-level custom nixpkgs import (newer, more flexible approach)

- **July 19, 2025**: Recent improvements (commits 55655e3, 4d5f37c, e19f799)
  enhanced flexibility but **inconsistency remained unfixed**

**Why it persisted:** The inconsistency was masked because module-level
`requiredRuntimePackages` usually overrode function-level defaults. Only
surfaced when users customized the kernel configuration.

## Solution

Updated `requiredRuntimePackages` to use the same `pkgs` instance that's
created with custom overlays, making the rust kernel architecturally
consistent:

```nix
# Before: Mixed approaches
requiredRuntimePackages = [config.nixpkgs.cargo ...];  # Uses config.nixpkgs
pkgs = import config.nixpkgs.path { overlays = ...; };  # Creates custom pkgs

# After: Consistent approach
pkgs = import config.nixpkgs.path { overlays = ...; };  # Create pkgs first
requiredRuntimePackages = [pkgs.cargo ...];            # Use same pkgs
```

This aligns the rust kernel with its own architectural pattern and eliminates
the dependency on `config.nixpkgs` being a package set.

## Testing

Verified the fix resolves evaluation errors in downstream projects that
configure rust kernels with custom nixpkgs paths and overlays.

Fixes tweag#564

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: jmmaloney4

modules/kernels/rust/default.nix

@@ -28,10 +28,19 @@
     name,
     ...
   }: let
+    # Create the custom pkgs with overlays applied first
+    pkgs = let
+      allOverlays = config.overlays ++ config.nixpkgs.extraOverlays;
+    in
+      import config.nixpkgs.path {
+        inherit system;
+        overlays = allOverlays;
+      };
+
     requiredRuntimePackages = [
-      config.nixpkgs.cargo
-      config.nixpkgs.gcc
-      config.nixpkgs.binutils-unwrapped
+      pkgs.cargo
+      pkgs.gcc
+      pkgs.binutils-unwrapped
     ];
     args = {inherit self system lib config name kernelName requiredRuntimePackages;};
     kernelModule = import ./../../kernel.nix args;
@@ -145,7 +154,7 @@
         kernelModule.kernelArgs
         // {
           inherit (config) evcxr;
-          # Import nixpkgs with all overlays applied
+          # Import nixpkgs with all overlays applied (same as used for requiredRuntimePackages)
           pkgs = let
             allOverlays = config.overlays ++ config.nixpkgs.extraOverlays;
           in