aced24b8

By: Michael Lynch <git@mtlynch.io>

Build and cache every package through flake checks

Make checks.all depend on every exported package so nix flake check covers the
complete build without shell-side target enumeration. Upload the aggregate
closure only after checks succeed, and resolve its system from Nix itself.

Backport refactoring-english-feedback fc756dc. Offline script tests cover option
forwarding, optional hooks, failures, and uploads invoked outside the checkout.