Update SAST.gitlab-ci.yml - Add SAST_GITLEAKS_ENTROPY_LEVEL

What does this MR do?

Properly propagates ENV variable SAST_GITLEAKS_ENTROPY_LEVEL within SAST CI template.

This bug prevented the variable to be passed to the child containers executed through the sast docker-in-docker context, causing the variable to be ignored. In effect, this prevents users from configuring their entropy detection

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/62179

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team