Fix registrationapp build by replacing ruby gems with npx for SASS

The registrationapp build is currently broken in github actions and on any fresh install due to lack of availability of the ruby gems that had previously been in use. This should be updated to build with node as was done in these examples:

pihcore: https://github.com/PIH/openmrs-module-pihcore/pull/266
labtrackingapp: https://github.com/PIH/openmrs-module-labtrackingapp/pull/24
appointmentschedulingui: https://github.com/openmrs/openmrs-module-appointmentschedulingui/pull/33