I'll answer your questions later, but have started the build
before I go out.

The build appears to be running something like

  until fc-something; do :; done

in an infinite loop, spamming the build log at best and slowing
down the build at worst.

Did you notice that too?

I get a lot of "Fontconfig error: No writable cache directories" which appeared after adding graphviz as a native input for building documentation. Is this what you are meaning?