Migrating csv data as array into multi-field
I figured it out.
The issue was the plugin. Rather than * array_build*, I should have been using the get plugin, as follows:

process:
  type:
    plugin: default_value
    default_value: census
  title: 'Country Name'
  field_country_code: 'Country Code'
  field_count:
    plugin: get
    default_value: 0
    source:
      - 1960
      - 1961
      - 1962
     .
     .
     .


That migrates the value of the array indexed by those years into the field_count field. And because the field has multiple cardinality, each row is written in as a new value.