assemblers/tar: use osbuild.api.arguments

Instead of reading the arguments from sys.stdin, which requires
that stdin is setup properly for that in the runner, use the new
api.arguments() method to directly fetch the arguments.

Also fix missing newlines between imports and methods to be more
PEP-8 complaint, where needed.

assemblers/org.osbuild.tar

@@ -19,10 +19,12 @@ Buildhost commands used: `tar` and any named `compression` program.
 """
 
 
-import json
 import subprocess
 import sys
 
+import osbuild.api
+
+
 SCHEMA = """
 "additionalProperties": false,
 "required": ["filename"],
@@ -39,6 +41,7 @@ SCHEMA = """
 }
 """
 
+
 def main(tree, output_dir, options):
     filename = options["filename"]
     compression = options.get("compression")
@@ -76,6 +79,6 @@ def main(tree, output_dir, options):
 
 
 if __name__ == '__main__':
-    args = json.load(sys.stdin)
+    args = osbuild.api.arguments()
     r = main(args["tree"], args["output_dir"], args["options"])
     sys.exit(r)