GitLab's secret detector has started generating two spurious Critial
severity leak detections due to scanning the `.git` directory of the
cloned project which, as part of the clone process, contains the
temporary GitLab CI token.

In the [upstream issue][1], customers are being advised to pin the
version to 7.9.0 to work around the issue.

[1]: https://gitlab.com/gitlab-org/gitlab/-/issues/560092#note_2672123492

fix(common-pipeline): pin secret detector image version

Closes #135

See merge request !172