Use CircleCI contexts for environment variables

[dbeatty10]

Description & motivation

Checklist

• [x] I have verified that these changes work locally
• [ ] I have updated the README.md (if applicable)
• [ ] I have added an integration test for my fix/feature (if applicable)

[dbeatty10]

Verified that CI is working for all but Databricks:

Will begin deleting project-specific environment variables in CircleCI starting with:

• GCLOUD_SERVICE_KEY
• BIGQUERY_SERVICE_ACCOUNT_JSON
• BIGQUERY_TEST_DATABASE

[dbeatty10]

Looks good:

Deleting all the project-specific CI_* environment variables in CircleCI next.

[dbeatty10]

That worked too:

Finally, will delete the following:

• REDSHIFT_*
• SNOWFLAKE_*

[dbeatty10]

That also worked, and will will leave the remaining environment variables for now.

[dbeatty10]

Added follow-up work in https://github.com/dbt-labs/dbt-external-tables/issues/181