[CI] Use pinned version of mac image

[florence-rolland]

Proposed change(s)

We are going to stop allowing people to target a Bokken image without specifying a tag. Only a handful of people are using it, and it gets in the way of using sourcegraph insights to estimate usage. Currently, no tag equals using the stable version of the image. I used a pinned version instead of stable because that tag is also going to disappear and I want to prevent you from having to do another maintenance in a few months.

Useful links (GitHub issues, JIRA tickets, forum threads, etc.)

JIRA Ticket https://jira.unity3d.com/browse/DSBKN-1034 Image version in the Bokken image catalogue http://images.bokken.cloud:8000/#/image/package-ci/mac/673349491555729414

Types of change(s)

• [ ] Bug fix
• [ ] New feature
• [ ] Code refactor
• [ ] Documentation update
• [x] Other (please describe) CI fix

Testing and Verification

Run affected CI pipeline on Yamato.

Test Configuration: Not applicable (CI environment)

• Unity Version: [e.g. Unity 2020.2.0f1]
• Unity machine OS + version: [e.g. Windows 10]
• ROS machine OS + version: [e.g. Ubuntu 18.04, ROS Noetic]
• ROS–Unity communication: [e.g. Docker]

Checklist

• [x] Ensured this PR is up-to-date with the dev branch
• [x] Created this PR to target the dev branch
• [x] Followed the style guidelines as described in the Contribution Guidelines
• [ ] Added tests that prove my fix is effective or that my feature works --> Not applicable
• [ ] Updated the Changelog and described changes in the Unreleased section --> Not applicable
• [ ] Updated the documentation as appropriate --> Not applicable

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