Derive dataset type from recipe object (rather than hardcoding)

[cisaacstern]

Now that @andersy005 completed pangeo-forge/pangeo-forge-recipes#437, we can actually do this:

pangeo-forge-orchestrator/pangeo_forge_orchestrator/routers/github_app.py

Lines 882 to 884 in fb5caaf

	# TODO: Derive `dataset_type` from recipe instance itself; hardcoding for now.
	# See https://github.com/pangeo-forge/pangeo-forge-recipes/issues/268
	# and https://github.com/pangeo-forge/staged-recipes/pull/154#issuecomment-1190925126

The hardcoding of dataset_type needs to be changed in two places:

1. pangeo-forge-orchestrator/pangeo_forge_orchestrator/routers/github_app.py

   Line 894 in fb5caaf

   dataset_type="zarr",

2. pangeo-forge-orchestrator/pangeo_forge_orchestrator/routers/github_app.py

   Line 1216 in fb5caaf

   dataset_type="zarr",

#198 will require this fix to actually work.