On MacOS it seems subsequent attempts to bind to the abstract Unix
socket fail without first explicitly removing it. This issue seems to be
MacOS-specific as it doesn't occur on an Ubuntu VM (using `lima`).

Fixes sigstore#738

Signed-off-by: Paul Thomson <thomsonp83@gmail.com>

MacOS doesn't have abstract unix domain sockets, and so creates a file
in the local dir which isn't removed when the server stops (due to the
handling of sockets beginning with '@')

This file needs to be removed on subsequent runs otherwise there's a
bind error

Signed-off-by: Paul Thomson <thomsonp83@gmail.com>