ci: update-maintainers fetch nixpkgs from flake.lock rev

We need a nixpkgs on NIX_PATH. Right now we have been using the latest
from channel. But, we can actually just fetch the nixpkgs from our
flake.lock by parsing the flake.lock.

Signed-off-by: Austin Horstman <khaneliman12@gmail.com>

.github/workflows/update-maintainers.yml

@@ -38,8 +38,14 @@ jobs:
         uses: actions/checkout@v4
         with:
           token: ${{ steps.app-token.outputs.token }}
+      - name: Get Nixpkgs revision from flake.lock
+        id: get-nixpkgs
+        run: |
+          echo "rev=$(jq -r '.nodes.nixpkgs.locked.rev' flake.lock)" >> "$GITHUB_OUTPUT"
       - name: Install Nix
         uses: cachix/install-nix-action@v31
+        with:
+          nix_path: nixpkgs=https://github.com/NixOS/nixpkgs/archive/${{ steps.get-nixpkgs.outputs.rev }}.tar.gz
       - name: Setup Git
         run: |
           git config user.name "${{ steps.user-info.outputs.name }}"