Fix node version to 16

Temporary workaround for:

Error message "error:0308010C:digital envelope routines::unsupported"

See e.g. discussion at https://stackoverflow.com/questions/69692842/error-message-error0308010cdigital-envelope-routinesunsupported

This is intended to solve the immediate problem of a broken build; when we are back up and running, I'll ticket an upgrade to more recent node.