pr flows

[ngalaiko]

No description provided.

[exe-dev-github-integration]

👋 Hey @ngalaiko, thanks for this PR!

Here is what I see you changing in the build workflow:

1. PR trigger — Adds pull_request so CI runs on PRs (not just master pushes)
2. Faster Nix install — Switches from DeterminateSystems/nix-installer-action to nixbuild/nix-quick-install-action (single-user mode so /nix can be cached)
3. Nix store caching — Introduces nix-community/cache-nix-action@v7 with hash-based keys, GC max 6G, and cache purge logic for stale entries
4. Build vs Push split — Build always runs (so PR CI validates builds); push and manifest only execute on non-PR events
5. Concurrency control — Cancels in-progress runs when a new push arrives on the same PR/branch

This looks great — CI will validate builds without pushing images on every PR commit. Let me know if you need any help with it!