Run NixCI checks for pull requests and default-branch pushes (#7) Run NixCI for pull requests and pushes to the default branch. `nix flake check` now builds every exported package through `checks.all`. The shared build wrapper accepts Nix options and invokes a separate cache-upload hook only after checks succeed. The hook uploads the aggregate check target and its package closure. Co-authored-by: Michael Lynch <git@mtlynch.io> Reviewed-on: https://codefloe.com/mtlynch/pointer-brother/pulls/7