fix: replace windows path separator with posix

[dpilch]

Description of changes

See https://github.com/aws-amplify/amplify-cli/issues/13289 for details. path.relative behaves differently on windows. https://nodejs.org/api/path.html#pathrelativefrom-to

This results in the generated path being invalid syntax in TypeScript/JavaScript.

This change replaces the current OS environment path separator (path.sep) with the posix path separator (path.posix.sep).

Issue #, if available

https://github.com/aws-amplify/amplify-cli/issues/13289

Description of how you validated changes

Nothing yet 😬

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)
• Breaking changes to existing customers are released behind a feature flag or major version update
• Changes are tested using sample applications for all relevant platforms (iOS/android/flutter/Javascript) that use the feature added/modified

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.