[fix] use posix to resolve relative path

[ignatiusmb]

Last adapter with relative that needs converting

Related #3163, #3200, #3211, #3212

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpx changeset and following the prompts. All changesets should be patch until SvelteKit 1.0

[changeset-bot]

🦋 Changeset detected

Latest commit: 2fcb0df

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/adapter-cloudflare	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[netlify]

✔️ Deploy Preview for kit-demo canceled.

🔨 Explore the source changes: 2fcb0df

🔍 Inspect the deploy log: https://app.netlify.com/sites/kit-demo/deploys/61d50e24a86876000770e431

[Rich-Harris]

The perils of assuming the vercel/cloudflare adapter will only run on vercel/cloudflare!