feat: add source flag to use different folder as source for building sites (#5334)

I do not expect the flag to be used much, but it enforces better code design, and prevents implicitly relying on cwd.

Just a general code style improvement really.

Co-authored-by: Darragh Elliott <me@delliott.net>
Co-authored-by: tobiasd <tobiasd@fsfe.org>
Reviewed-on: #5334
Co-authored-by: delliott <delliott@fsfe.org>
Co-committed-by: delliott <delliott@fsfe.org>

fsfe.org/internal/subdir.py

@@ -15,7 +15,7 @@ from lxml import etree
 logger = logging.getLogger(__name__)
 
 
-def run(languages: list[str], processes: int, working_dir: Path) -> None:  # noqa: ARG001 # We allow unused args for subdirs
+def run(source: Path, languages: list[str], processes: int, working_dir: Path) -> None:  # noqa: ARG001 # We allow unused args for subdirs
     """
     Internal subdir preparation
     """