Ensure final asset directory exists before writing cached files

[oliverlynch]

Changes

Create the asset output folder before writing from cache.

If the first optimised image asset is cached astro re-optimises it as the cached file could not be written to the output directory, which is created after the cache step. Instead create the output directory first to prevent the unnecessary redownload and reprocessing of the first image asset.

Testing

Tested with the astro base template with a single remote image with 3 densities added, and successfully ran pnpm --filter astro run test.
Improved test site optimised image generation time on rebuilds from ~280ms to ~1ms, although this is an extreme case.

Docs

N/A: Minor fix in caching behaviour that should conform to existing user expectations

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[Princesseuh]

Looks great to me! Thank you very much for contributing this fix!