Drop NixCI dependency edges to doNotBuild jobs
## Problem
The newer NixCI validates build-dependency edges and rejects any edge whose
endpoint is a job it does not build (i.e. a job listed in `doNotBuild`). The
manual dependency graph in `nix-ci.nix` referenced **35** such jobs, so NixCI
now shows a `build-dependency configuration error`:
- `checks.x86_64-linux.gammaloop-doctest`, `checks.x86_64-linux.gammaloop-nextest-binaries`
- every `crate-*` and `crate-deps-*` artifact (all `doNotBuild`)
- `packages.x86_64-linux.default`, `workspaceBuildArtifacts`,
`workspaceDoctestCargoArtifacts`, `gammaloop-llvm-coverage`,
`nix-ci-check-gammaloop-nextest`
These jobs exist in the flake but are excluded from CI via `doNotBuild`, so
ordering edges to/from them are meaningless — NixCI never schedules them.
## Fix
Keep constructing the full crate/artifact DAG (so the drift and cycle asserts
stay meaningful), then run the emitted graph through a new
`buildableDependencies` filter that drops every edge touching a `doNotBuild`
job. Only ordering between jobs NixCI actually builds survives.
Result: **162** meaningless edges dropped, **96** edges between built jobs kept
(workspace-hack cache root, the test-support DAG, and nextest-archive ordering
— all correctness-neutral scheduling hints).
## Testing
- `nix eval` of `nix-ci.nix` succeeds.
- An assertion over the emitted graph confirms **0** remaining edges reference
a `doNotBuild` job (down from 35 endpoints / 162 edges).