mkosi: Only lower device timeout instead of all timeouts

We only really care about lowering the device timeout so we get to
a shell faster when the root device doesn't appear so let's only
lower that timeout instead of lowering all default timeouts.

.github/workflows/mkosi.yml

@@ -87,7 +87,6 @@ jobs:
 
         [Content]
         Environment=CI_BUILD=1
-                    DEFAULT_TIMEOUT_SEC=180
                     SLOW_TESTS=true
 
         [Output]
@@ -95,6 +94,8 @@ jobs:
                                systemd.journald.max_level_console=debug
                                # udev's debug log output is very verbose, so up it to info in CI.
                                udev.log_level=info
+                               # Root device can take a long time to appear, so let's bump the timeout.
+                               systemd.default_device_timeout_sec=180
 
         [Host]
         ExtraSearchPaths=!*