chore: use a post-build hook to sign binaries

Description

The default binary_signs requires that a separate signature needs to be generated and uploaded. Simplify this by using a post-build hook to code sign (https://goreleaser.com/customization/builds/hooks/?h=hooks).

This also eliminates the extraneous SHA-256 checksum files.

Related Issues

How has this been tested?

See https://gitlab.com/gitlab-org/cli/-/jobs/11915092839.

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