Fix assets copying

[CustomEntity]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

I checked the Test checkbox because I couldn't find a way to test for it.

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

This pull request addresses the issues encountered with the Nest CLI assets property on Windows platforms (#2397 and #2429), specifically related to the path format causing file copying to fail. The modifications ensure compatibility across different operating systems and correct the handling of Windows path formats.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[CustomEntity]

@kamilmysliwiec Is it possible to take a look? 😄