Prepare for `brew service` usage in homebrew formula

[maxzinkus]

Currently, the only way to get the launchctl launch agent to auto-start instances as login is to build from source and use make install && make agent.

brew supports installing plist service files in formulae (refer to brew service). It requires the plist file to be available within the project so that it can be installed:

prefix.install_symlink "file.plist" => "#{plist_name}.plist"

This PR:

• changes make install to build the plist file once alpine is installed into $PATH
• removes make agent

Once this is merged, I'll open a PR on the macpine formula to install the plist file into Cellar per above, so that brew service will notice it.