Argument prefixes not compiled correctly when using javascript expressions

[arunisaac]

Something like #:run ("-pop" "$(inputs.population.nameroot)") is compiled to

arguments:
  - position: 3
     valueFrom: "-pop"
   - position: 4
      valueFrom: $(inputs.population.nameroot)

It should rather be compiled to

- position: 3
   prefix: "-pop"
   valueFrom: $(inputs.population.nameroot)

Thanks to glassofwhiskey for reporting this in the Matrix CWL room!