ci: Add --fix flag to eslint

Running ./.gitlab-ci/run-eslint gives messages like:

64 errors and 0 warnings potentially fixable with the --fix option.

but before this change, --fix would not actually do anything.

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I am sure I'm not the only fan of fixing things automatically rather than manually. This change saved me from making 64 boring manual adjustments, and who knows how many it will save for other people in the future.