CLI hangs if EPICS envvars are set

[tpoliaw]

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