pipeline: split out downloading from building

Separate the downloading form the actual building, so that in the
future one could only download artifacts needed to build images.

osbuild/main_cli.py

@@ -134,6 +134,9 @@ def osbuild_cli():
     try:
         output_directory = args.output_directory
         with ObjectStore(args.store) as object_store:
+
+            manifest.download(object_store, args.libdir)
+
             r = manifest.build(
                 object_store,
                 monitor,

osbuild/pipeline.py

@@ -344,12 +344,13 @@ class Manifest:
         self.sources.append(source)
         return source
 
-    def build(self, store, monitor, libdir, output_directory):
-        results = {"success": True}
-
+    def download(self, store, libdir):
         for source in self.sources:
             source.download(store, libdir)
 
+    def build(self, store, monitor, libdir, output_directory):
+        results = {"success": True}
+
         for pl in self.pipelines.values():
             res = pl.run(store, monitor, libdir, output_directory)
             results[pl.id] = res