fix: don't use scope for trivy registry login command

[DmitriyLewen]

Description

DockerHub returns error when we use scope (see #8386 ).
So we need to use empty scope to fix this error.

We can confirm that it works for DockerHub, GHCR (#8386 (comment)).

Related issues

• Close trivy registry login fails for docker.io with "unknown resource type" error #8386

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[knqyf263]

I tested with ECR.

$ aws ecr get-login-password | ./trivy registry login --username AWS --password-stdin <0123456789>.dkr.ecr.eu-central-1.amazonaws.com