bug: Set the correct cache headers for static files

[Jarrku]

The cache header values are currently passed as seconds instead of ms, which the library that @fastify/static uses expects (https://www.npmjs.com/package/send#maxage).

You can pass a string value which is correctly parsed in the the right amount of ms.

There's another issue I've noticed, the assetsBuildDirectory files do not get handled by the stricter handler (with immutable).

I've tried to set the prefix to /build/ so they're correctly picked up, but I receive errors for double registration of routes then.
Unsure how to properly resolve that issue.

[mcansh]

thanks for the fix @Jarrku 🙏 - i'll take a look at the build prefix issue in a bit