bug 1456620 - move check_if_partners_enabled call. r=nthomas

Because the transforms are generators, we actually call them from the bottom up. The previous transforms don't get called until the `for task in tasks:` or `for job in jobs:` in the following transform.

Moving the `check_if_partners_enabled` transform to the end means we never try to access `config.params['release_partner_config'].values()` in `add_command_arguments` when `release_partner_config` is None. Otherwise, we hit errors when we run taskgraph-gen.py.

MozReview-Commit-ID: Ho2odPL9FxS

--HG--
extra : rebase_source : 51406b39c358ff99690e073920a2e3f66cb39c83
extra : source : 02136f9beec0726098d9263f84f46244bd454b9f

taskcluster/taskgraph/transforms/partner_repack.py

@@ -15,8 +15,6 @@ from taskgraph.util.partners import check_if_partners_enabled
 
 transforms = TransformSequence()
 
-transforms.add(check_if_partners_enabled)
-
 
 @transforms.add
 def resolve_properties(config, tasks):
@@ -61,3 +59,8 @@ def add_command_arguments(config, tasks):
         }
 
         yield task
+
+
+# This needs to be run at the *end*, because the generators are called in
+# reverse order, when each downstream transform references `tasks`.
+transforms.add(check_if_partners_enabled)