It turns out that when compiling with MinGW you should also quote arguments to
spawn. This also revealed two other problems: one where I inadvertently removed the closing
" from the kernel_library etc. arguments; the other where the arguments to the linker are already quoted and do not need to be quoted again when creating the command line for
Sorry that this all happens in separate MRs, because I first didn't realise the environment on the build server was different, and then wasn't sure how to reproduce it exactly, it was a bit tricky... The platform test runs OK with these changes (https://gitlab.science.ru.nl/clean-and-itasks/clean-platform/-/jobs/109873), so hopefully this is the last change for the time being.