hotfix: hardcoded filepath is now unwriteable in our ECS tasks

https://carnegielearning.atlassian.net/browse/CICD-559 `src/api/assets/files` is hardcoded in analysisService as the dir to write to but in new setup the only writeable volume is `/tmp`, so we need to update this path. while doing so I'll create an env var so this isn't hardcoded.