fix(dbus): Block on creation of zbus connection to avoid deadlock

A fix for the issue reported in
https://github.com/pop-os/cosmic-comp/pull/2450.

Using `block_on` in some places for zbus calls is fine (that's what
zbus's blocking API does; it has its own executor for background tasks),
but this is problematic with `async_once_cell`. If we also use that in
the calloop async executor.

I think ideally we should avoid blocking the main thread on any async
tasks, but for now we can just block in initialization here.

Cargo.toml

@@ -83,7 +83,6 @@ logind-zbus = { version = "5.3.2", optional = true }
 futures-executor = { version = "0.3.32" }
 futures-util = "0.3.32"
 cgmath = "0.18.0"
-async-once-cell = "0.5.4"
 
 [dependencies.id_tree]
 branch = "feature/copy_clone"