Fix the taskcluster code that handles GitHub releases

In order to enable the nightly builds, we made `get_version()` input the
decision parameters.  However, this clearly isn't going to work in the
`get_decision_parameters()` function.

taskcluster/app_services_taskgraph/__init__.py

@@ -10,7 +10,7 @@ import re
 
 from taskgraph.parameters import extend_parameters_schema
 from . import branch_builds
-from .build_config import get_version
+from .build_config import get_version_from_build_config
 
 PREVIEW_RE = re.compile(r'\[preview ([\w-]+)\]')
 
@@ -49,7 +49,7 @@ def get_decision_parameters(graph_config, parameters):
                     head_tag
                 )
             )
-        version = get_version(graph_config.params)
+        version = get_version_from_build_config()
         # XXX: tags are in the format of `v<semver>`
         if head_tag[1:] != version:
             raise ValueError(

taskcluster/app_services_taskgraph/build_config.py

@@ -32,7 +32,7 @@ def get_components():
 
 
 def get_version(params):
-    version = _read_build_config()["libraryVersion"]
+    version = get_version_from_build_config()
     preview_build = params.get('preview-build')
     if preview_build == 'nightly':
         components = version.split('.')
@@ -44,6 +44,9 @@ def get_version(params):
     else:
         return version
 
+def get_version_from_build_config():
+    return _read_build_config()["libraryVersion"]
+
 def get_extensions(module_name):
     publications = _read_build_config()["projects"][module_name]['publications']
     extensions = {}