Store alongside attrpath + nixpkgs revision

- unknown (for archs not built by hydra, or nixpkgs revisions that were
  in-between two hydra evals)
- manual-cannot-be-made-to-build (eg. if upstream doesn't support it)
- build-broken
- builds
- passes-tests (passthru.tests)
- manual-tests-passed (for manually testing)

if unknown, fetch last known build status

if passes-tests, look if history looks like (passes-tests)* manual-tests-passed,
and if so say how long it's been (if not, say for how long it's been
passes-tests since the last lower-level build)

if build-broken, look if history looks like (build-broken)*
manual-cannot-be-made-to-build, and if so say how long it's been (if not, say
for how long it's been build-broken since the last higher-level build)

database should be auto-filled by hydra + committers could also manually fill
statuses, esp.:
- build-broken -> manual-cannot-be-made-to-build
- passes-tests -> manual-tests-passed
- unknown -> any other level (so exotic archs know what works)

improving on an arch would basically be taking builds that are build-broken but
don't have a manual-cannot-be-made-to-build history, and trying to either fix
them or to mark them as manual-cannot-be-made-to-build

Further ideas:
- hydra could compute a probability of success based on the history of the
  build, to change the likelihood it has of rebuilding a package, thus
  economizing some compute
- user-facing nix could use said probability (instead of the above “last known
  status + metadata” idea) to evaluate working likelihood