Allow restricted administrators to add executables in the custom 'Command' task

A full administrator (which is only Atlassian Support in Bamboo OnDemand) has the ability to add new executables to the list of available executables in the Command task. Restricted administrators (i.e. customer admins) do not have this ability.

Considering restricted admins can use elastic custom images, it seems that they should have the flexibility to add their own executables here.