.gitlab-ci.yml: Enable colors on the CI
BuildStream is unable to detect whether output has support for ANSI color codes automatically when running in a CI and not connected to a real tty.
It is however typical of CI systems to support the ANSI color escape codes (but not other terminal escape sequences), for this reason BuildStream has an explicit --colors option to force colors to be enabled in scenarios where we cannot detect that it is supported.
This patch causes the CI to use it.