Fix all links to nightly releases (#4519)

Nightly (a.k.a. development) releases are published to knative-nightly,
and do not use the org name. This has not changed since 2019-05-17:
09f2b26361/scripts/release.sh (L96)

Thank you @andersonsoares-tomtom for reporting it via
https://github.com/knative/eventing/issues/5930

Fixes knative/eventing#5930

Signed-off-by: Gerhard Lazu <glazu@vmware.com>

hack/macros.py

@@ -27,10 +27,9 @@ def define_env(env):
         """
         version = os.environ.get("KNATIVE_VERSION")
         if version == None:
-            return 'https://storage.googleapis.com/{org}-nightly/{repo}/latest/{file}'.format(
+            return 'https://storage.googleapis.com/knative-nightly/{repo}/latest/{file}'.format(
                     repo=repo,
-                    file=file,
-                    org=org)
+                    file=file)
         else:
             return 'https://github.com/{org}/{repo}/releases/download/{version}/{file}'.format(
                     repo=repo,