Add support for arm64 mattermost.

Summary

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

Proposal

I have an arm64 server that can't run the built-in mattermost executable, as it is the amd64 version. This will fix it so it downloads the correct executable for the correct machine.

References

There is a completed merge request for this: !5932 (closed)