fix: add checksums to signing

Description

Previously the publish process failed while trying to upload signatures after signing. Generate a SHA-256 checksum for each artifact.

Related Issues

• Sample job: https://gitlab.com/gitlab-org/cli/-/jobs/11363961610
• Previously failing job: https://gitlab.com/gitlab-org/cli/-/jobs/11360221076

How has this been tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap