Add support for arm64 mattermost.

What does this MR do?

This fixes the mattermost package and allows it to download the arm64 variant on arm64-based systems.

Related issues

Closes #6694

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com (I'm 13, I don't have a credit card, so I don't have a CI-validated account)
• Pipeline is green on the dev.gitlab.org mirror for this branch if the change is touching anything besides documentation or internal cookbooks (I'm 13, I don't have a credit card, so I don't have a CI-validated account)
• trigger-package has a green pipeline running against latest commit
• Correct sha checksums and download URLs.

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated (None needed)
• Tests added (I don't know how, and I don't know if we need them)
• Integration tests added to GitLab QA (I don't know how, and I don't know if we need them)
• Equivalent MR/issue for the GitLab Chart opened (Opening an issue)