chore: manage Dagger via Nix flake (0.21.8)

[jpmcb]

Align Dagger to the tapes #273 pattern: add a dagger flake input (github:dagger/nix), put the CLI in the default devShell, and run PR-check workflows through nix develop --command dagger instead of DAGGER_VERSION + dagger-for-github.

• Pin ghcontrib to 45e997aec734ec4875ff57bebed2317e1bd49185
• Install Nix with determinate-nix-action v3.21.8 and magic-nix-cache-action v14
• Verified nix develop --command dagger version → v0.21.8

[greptile-apps]

Greptile Summary

Updates the pull-request workflow from Dagger 0.20.8 to 0.21.8 and upgrades both installation steps from dagger-for-github v8.2.0 to v8.4.1.

• Applies the new Dagger engine version consistently across both PR validation jobs.
• Uses the updated GitHub Action release in both installation steps.

Confidence Score: 5/5

The PR appears safe to merge with no identified actionable regressions.

The upgraded engine accepts the older engine version declared by the invoked remote module, and the module’s standard container operations remain compatible with the new engine.

Important Files Changed

Filename	Overview
.github/workflows/pr.yaml	The workflow consistently upgrades the Dagger engine and installer action; no actionable compatibility or security regression was established.

_{Reviews (1): Last reviewed commit: "chore: bump Dagger to 0.21.8" | Re-trigger Greptile}