fix: use relative path when resolving nested stack location

[mndeveci]

Which issue(s) does this change fix?

N/A

Why is this change necessary?

When resolving location of the nested stack template in InfraSyncExecutor, it should resolve relative to the parent template which is referencing to the nested one.

How does it address the issue?

It resolves by prepending parent folder of the current template.

What side effects does this change have?

N/A

Mandatory Checklist

PRs will only be reviewed after checklist is complete

• Add input/output type hints to new functions/methods
• Write design document if needed (Do I need to write a design document?)
• Write/update unit tests
• Write/update integration tests
• Write/update functional tests if needed
• make pr passes
• make update-reproducible-reqs if dependencies were changed
• Write documentation

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