ci: Add `go fmt` check to the CI pipeline

Description

Per our code review guidelines:

Enforcing and improving code style should be primarily done through automation instead of review comments.

The checking of formatting is easily automated. This MR adds a new CI job to check that all files are properly formatted, so that formatting issues can be called out by the pipeline instead of by code reviewers.

Related Issues

N/A

How has this been tested?

• Using the CI pipeline on this merge request
• Using a branch with unformatted files: https://gitlab.com/gitlab-org/cli/-/jobs/3328458315

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)