Fix intermittent mkdir race when generating with multiple workers

[mynetx]

When generating with multiple workers, a page would intermittently fail with:

[✘] /fr (mkdir(): File exists)

Two workers can race on the same output directory: one checks that it doesn't exist, another creates it in the meantime (e.g. while writing a child page like /fr/stories/...), and the first worker's mkdir then throws. Since pages are shuffled across workers, it's timing-dependent — on a small 13-page multisite it failed 5 out of 20 runs with --workers=4.

This forces the directory creation, so an already-existing directory is treated as success. Genuine failures (permissions etc.) still surface through the put() right after. With the fix, the same 20-run loop passes cleanly.

Co-Authored-By: Claude noreply@anthropic.com