Create the data directory before writing the sheet

SheetFetcher.save() opened paths.sheet_html without creating its parent,
so a fresh install failed with FileNotFoundError on the first fetch --
Paths only computes locations, and this is usually the first write to
the data root. Matches how every other writer in the pipeline behaves.

src/apworld_tester/pipeline/sheet/fetcher.py

@@ -51,6 +51,7 @@ class SheetFetcher:
     def save(self, content=None):
         """Write the tab's HTML, returning where it landed."""
         content = self.fetch() if content is None else content
+        os.makedirs(os.path.dirname(self.paths.sheet_html), exist_ok=True)
         with open(self.paths.sheet_html, "wb") as handle:
             handle.write(content)
         return self.paths.sheet_html