Add environment variable for gathering and surveying per stage

### Context

We don't want both prod and staging to process the same amount of experiment accessions per cycle. We can fix this by making envvars for each stage and then define settings from those values.

### Problem or idea

- add envars to terraform - variable.tf / environment.tpl
- add values to environment files - environment/stage.tfvars
- update cron job to utilize envars
- add setting that reads from envars
- update `gather_weekly_accessions` to use the new setting
- update foreman + api to use setting

If you need an example you can look for any setting that is defined by an env var that is not initially defined in github secrets.