Fix homebrew tap automation

[kenjenkins]

In #325 I split up the release process to build separately for macOS and Windows, in order to support platform-specific cgo code for integrating with the OS trust store on macOS and Windows. The macOS builds run on a macOS runner, the Windows builds run on a Windows runner, and the existing GoReleaser job handles just the Linux (and FreeBSD) builds.

However, I did not consider that GoReleaser also updates the https://github.com/pomerium/homebrew-tap repository, so removing the macOS configuration from GoReleaser ended up also dropping macOS from the automated homebrew formula update.

Let's see if we can fix the automation to avoid this:

• Is there a way to configure GoReleaser to include the macOS binaries even though it is no longer responsible for building them?
• Or can we split out the homebrew formula update into a separate task, apart from GoReleaser?