$FLATPAK_BUILDER_N_JOBS was added in 0.10.10 for cases like ours. We
already require 0.10.10 because the manifest is a YAML file.

While we're here, we can arrange to pass -j when building Bustle
itself. Previously it was built serially, unlike all Haskell
dependencies.

We can also avoid open-coding paths for --extra-{lib,include}-dirs, for
no functional change.