fix: use ephemeral directories to serve functions

[eduardoboucas]

I noticed that I would get errors in functions when using the Vite plugin and making multiple requests to the same function. I realised that this is because, unlike the CLI, in the Vite plugin we're not watching files for changes. We're just calling the handle() function on requests and that will cause functions to be built with the latest state of their source files.

In practice, this meant that we could have multiple function builders writing to the .netlify/functions-serve directory, which was breaking things.

This PR introduces two changes:

1. We stop building all functions automatically, and instead we lazily build only the function that matches the incoming request
2. We make each function builder write to an ephemeral directory inside .netlify/functions-serve, so that multiple builders are not destroying each other's working directories; these directories are cleaned up after each invocation

It depends on netlify/build#6353, so it's expected that tests will fail until it has been merged and released, but I wanted to have the PR up for review sooner rather than later.

[serhalp]

LGTM