nix: replace deprecated pkgs.system with stdenv.hostPlatform.system

[amaanq]

Description

This pr fixes a deprecation warning that nix emits, as well as updating a package's name as it was renamed

Fixes # (issue)

N/A

Type of change

Please delete options that aren't relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

I tested and eval'd this locally and it still works

Checklist

• Updated documentation if needed
• Tests added/amended
• bazel test //... passes locally
• PR is contained in a single commit, using git amend see some docs

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This change is 

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