Resolve "sha1sum is required to build gitaly"

What does this merge request do and why?

Add sha1sum command from the Brewfile, which is required to update/build gitaly.

How to set up and validate locally

• Run brew bundle to install md5sha1sum package
• Run make gitaly-update
• Ensure there are no sha1sum errors

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Closes #1753 (closed)