Make go mock an explicit dependency in tools

[eleftherias]

Summary

Prior to this commit, the go mock dependency was implicitly added by the openFGA dependency.
The latest openFGA update removed the implicit dependency and caused our CI to fail (see https://github.com/stacklok/minder/actions/runs/10347230154/job/28637107802?pr=4112)

Change Type

Mark the type of change your PR introduces:

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

Outline how the changes were tested, including steps to reproduce and any relevant configurations.
Attach screenshots if helpful.

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[stacklok-cloud-staging]

Minder Vulnerability Report ✅

Minder analyzed this PR and found no vulnerable dependencies.

Vulnerability scan of be6b712c:

• 🐞 vulnerable packages: 0
• 🛠 fixes available for: 0

[coveralls]

coverage: 53.891% (-0.008%) from 53.899%
when pulling be6b712 on eleftherias:fix-tools-deps
into e438848 on stacklok:main.