Fix pynn-brainscales build 2nd try

• This should fix the error about an ambiguous template instantiation for struct halco::common::detail::limits<halco::hicann_dls::vx::v1::SynapseOnSynapseRow, void> which isn't triggered on newer g++s. We dropped support for old v1 hardware, this was planned anyways.
  • side effect: we build fewer targets ⇒ faster build 🎉
• add --keep build flag to keep building after the initial fail ⇒ let's try to find all problems in one go.