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.lock

@@ -242,12 +242,6 @@ dependencies = [
  "pin-project-lite",
 ]
 
-[[package]]
-name = "async-once-cell"
-version = "0.5.4"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4288f83726785267c6f2ef073a3d83dc3f9b81464e9f99898240cced85fce35a"
-
 [[package]]
 name = "async-process"
 version = "2.5.0"
@@ -835,7 +829,6 @@ name = "cosmic-comp"
 version = "1.0.0"
 dependencies = [
  "anyhow",
- "async-once-cell",
  "bitflags 2.11.0",
  "calloop 0.14.4",
  "cgmath",