Multi-config CI wasn't actually trying multiple configurations. This
fixes that issue and uses a less fragile method of producing fusesoc
options. They are generated once and stored in a variable so we cannot
accidentally break one or more steps by using an incorrect
ibex_config.py command in one step whilst using a correct
ibex_config.py in the display step (which is also intended to check the
ibex_config.py command is correct).
