fix: Do not remove data

[Duologic]

It is possible to reference the data object in the environment spec, however this doesn't
work if we remove it. Hiding should be sufficient to get the speed boost from lazy
loading.

The example references this.data outside the data: {} block, which is perfectly valid although I would discourage that, I'd rather suggest that data receives information from the environment it belongs to than the other way around.

Example:

local clusterEnvConfig = { namespace: 'a' };
{
  local this = self,
  apiVersion: 'tanka.dev/v1alpha1',
  kind: 'Environment',
  metadata: {
    name: 'environments/my-app/local',
  },
  spec: {
    namespace: this.data.app._config.namespace,
  },
  data: {
    app: { _config:: { namespace: 'b' } } +
         {
           _config+:: clusterEnvConfig,
           app+: {
             _config+:: {
               image: 'localhost:5000/my-app',
             },
           },
         },
  },
}

ref: https://github.com/nabadger/tanka-export-issue (reported by @nabadger)