chore(ci): remove unused env variables for IaC / SCA scans

[gg-mmill]

Context

Cleanup

What has been done

Remove unused env var in the scan sca and scan iac jobs

Validation

• checks in CI: https://github.com/GitGuardian/ggshield/actions/runs/9581923357/job/26419785059#step:3:294 and https://github.com/GitGuardian/ggshield/actions/runs/9581923357/job/26419789565#step:3:298

PR check list

• [ ] As much as possible, the changes include tests (unit and/or functional)
• [ ] If the changes affect the end user (new feature, behavior change, bug fix) then the PR has a changelog entry (see doc/dev/getting-started.md). If the changes do not affect the end user, then the skip-changelog label has been added to the PR.