support homebrew in system updater

### Feature Description

I have homebrew installed and want the binary to be detected as well.

### Use Case

depending on distro some binaries are only available on homebrew, some just like to use binaries that dont require root. Convenience.

### Compositor(s)

Niri

### If Other, please specify

_No response_

### Proposed Solution

detect the `brew` binary

### Alternatives/Existing Solutions

_No response_

Start by locating the system updater and the existing binary-detection entry point, then compare how available binaries are discovered. The work is done when the updater also detects the brew binary and the resulting update flow handles Homebrew-installed binaries as described.

Written by the indexing model from the issue text.