refactor: use single entry for SDist builds

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

unit_test/utils.py

@@ -1,3 +1,5 @@
+from pathlib import Path
+
 from cibuildwheel.options import CommandLineArguments
 
 
@@ -8,8 +10,8 @@ def get_default_command_line_arguments() -> CommandLineArguments:
     defaults.allow_empty = False
     defaults.archs = None
     defaults.config_file = ""
-    defaults.output_dir = None
-    defaults.package_dir = "."
+    defaults.output_dir = Path("wheelhouse")  # This must be resolved from "None" before passing
+    defaults.package_dir = Path(".")
     defaults.prerelease_pythons = False
     defaults.print_build_identifiers = False