Use SECRET_ENV_PREFIX from dbt_common instead of dbt_core

[emmyoop]

Housekeeping

• I am a maintainer of dbt-core

Short description

We duplicate teh expected value of SECRET_ENV_PREFIX in dbt-core and dbt-common. We should stop.

Acceptance criteria

SECRET_ENV_PREFIX is not defined in dbt-core and only imported by dbt-common

Suggested Tests

existing tests need to be updated, no additional tests needed

Impact to Other Teams

None

Will backports be required?

No

Context

Defining this constant in 2 places creates risk that it will get out of sync.