dev/ci: convert root_dir to abs path
Created by: bobheadxi
follow-up to https://github.com/sourcegraph/sourcegraph/commit/cc58dbe08e7d4db851ca54f6330896b5ad28f375
Created by: bobheadxi
follow-up to https://github.com/sourcegraph/sourcegraph/commit/cc58dbe08e7d4db851ca54f6330896b5ad28f375