gitlab-ci: add --force-color to twister invocation

[mniestroj]

GitLab CI handles color escape sequences without any issues, however automatic detection does not work due to output stream of running commands not being a tty.

Add --force-color, so that twister output will be colorized.

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 5e9fff7):

https://golioth-zephyr-sdk-doxygen-dev--pr310-gitlab-ci-force-4zoyyyhh.web.app

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