Currently, the hydra.nixos.org queue contains 1000s of Darwin builds
that all depend on a stdenv-darwin that previously failed. However,
before, first createStep() would construct a dependency graph for each
build, then getQueuedBuilds() would discover that one of the steps had
failed previously and discard all those steps. Since the graph
construction involves a lot of uncached calls to isValidPath(), this
took several seconds per build.
Now createStep() detects the previous failure right away and bails
out.
177bf25d64