[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
• 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

• Ensured this PR is up-to-date with the dev branch
• Created this PR to target the dev branch
• 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

Other comments