[ci] Expose SB_BUILD_PREFIX to CI
When trigering the CI, the user will have this variable prefiled. For more information see https://docs.gitlab.com/ee/ci/pipelines/index.html#prefill-variables-in-manual-pipelines
GitLab update will be applied on June 28th, between 8am and 10:30am, EDT (UTC-04:00). This site may have intermittent delays during that time.
When trigering the CI, the user will have this variable prefiled. For more information see https://docs.gitlab.com/ee/ci/pipelines/index.html#prefill-variables-in-manual-pipelines