ci: consolidate nix setup and drop nix develop --command wrappers

[jonstacks]

What

While working on our setup up Copilot Agent, I found that we can use direnv export gha to put all of our nix provided tools on the path. I also noticed that we were installing nix and the cache action in many places. This makes it harder to update the versions of these actions across all of our workflows.

How

• Consolidate install nix, setup up the cache, and downloading and using direnv to a single place and use it in our other workflows.
• Since we are now using direnv export gha, we no longer need to use nix develop --command across our actions

Breaking Changes

No, this should only impact CI

[Copilot]

Pull request overview

This PR consolidates Nix + cache + direnv setup into a reusable composite GitHub Action and updates CI workflows to rely on direnv export gha so steps can run make … directly without nix develop --command … wrappers.

Changes:

• Add a new composite action (.github/actions/nix-setup) to install Nix, configure the magic cache, and export the flake devShell environment into subsequent steps.
• Update multiple workflows to use the new nix-setup action and replace nix develop --command make … with make ….
• Update the build-and-test composite action to run make directly (assuming the environment is already prepared by nix-setup).

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
.github/workflows/helm_release.yaml	Use centralized nix-setup; run Helm setup via make directly.
.github/workflows/generate-full-install-manifests.yaml	Use centralized nix-setup; drop nix develop wrapper (but workflow trigger/checkout config needs correction).
.github/workflows/copilot-setup-steps.yml	Replace inline Nix/cache/direnv steps with centralized nix-setup.
.github/workflows/ci.yaml	Use centralized nix-setup across jobs; run make/go commands directly.
.github/actions/nix-setup/action.yaml	New composite action implementing the consolidated Nix + cache + direnv setup (currently has a YAML structure issue).
.github/actions/build-and-test/action.yaml	Remove nix develop --command wrappers and run make targets directly.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.52%. Comparing base (7b8fb26) to head (03a5632).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #795   +/-   ##
=======================================
  Coverage   51.52%   51.52%           
=======================================
  Files         101      101           
  Lines       11188    11188           
=======================================
  Hits         5765     5765           
- Misses       5002     5004    +2     
+ Partials      421      419    -2     

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