fix: pin trivy to 0.54.0

As explained in #128 (closed), following the 0.55.0 release of trivy our job has been failing to initialise remote Terraform modules.

This commit pins trivy to version 0.54.0 and adds it to the renovatebot ignore list while we work out what changes need to happen before we can bump the version further.

The issue tracking the progress of the long term fix is #131 (closed)