Verify brew installations on/before release

[wagoodman]

What would you like to be added:
Do one of the following:

• Add post-release automation that attempts to install syft via our brew tap and verify the installation + version.
• Add a goreleaser brews.test section to test if an installation worked before release (preferred if my understanding of the feature is correct).

Why is this needed:
We recently ran into a problem with goreleaser where the checksums of the asset referenced in the brew formula was wrong, preventing installations. The first indication of a problem was when users begin to report problems in slack and issues. It would be ideal to prevent this by testing before the release or at the very least get notified on slack after a release if a validation fails.

Related to #519

[jonasagx]

It would great to have it tested/verified in a CI box running macOS, or maybe linux since Homebrew supports linux