CLI hangs if EPICS envvars are set

If blueapi is used while the epics environment variables are set but the IOCs are not running (so nothing connects), the command hangs for ~30s before exiting.

From the root of the project
```
$ source tests/system_tests/.env
$ uv run blueapi
CAC: Unable to connect because "Connection refused"
Please invoke subcommand!
CAC TCP socket shutdown ERROR was Transport endpoint is not connected
$
```

The command does exit eventually but takes ~30s.

The variables set by the env file are
```
export EPICS_CA_NAME_SERVERS=127.0.0.1:9064
export EPICS_PVA_NAME_SERVERS=127.0.0.1:9075
export EPICS_CA_ADDR_LIST=127.0.0.1:9064
```

This feels like it's another occurrence of #1147 but not fixed by the mock signal created last time