fix(pre-commit): certdir variable must be an empty string

This fix is required to allow our pre-commit job to use docker-in-docker properly now that it is executing on the GKE-based runners.

Example job using the variable and docker-in-docker successfully - https://gitlab.developers.cam.ac.uk/uis/devops/experiments/rk725-demo/webapp-test/-/jobs/3162675

Part of https://gitlab.developers.cam.ac.uk/uis/devops/devhub/gitlab-runner-infrastructure/-/issues/88