Mode = prod + system_libs = false produces broken release

[keynslug]

While working on #902 and testing different combinations of options I found out that combination of {mode, prod} + {system_libs, false} produces broken releases that can't be started.

I suppose this is because the script currently assumes that system libraries are located in the same place as the runtime system, and under {mode, prod} this evaluates to the release's libdir where system libraries are obviously missing given {system_libs, false}.

Followup to #902 (comment).