fix(vitest): auto-enable "github-actions" only where users didn't configure reporters

[hi-ogawa]

Description

• follow up feat(vitest): add github actions reporter #5093

Based on the discussion on discord https://discord.com/channels/917386801235247114/1123254125191241868/1205400484530225214, this PR changes github-actions to be auto-enabled only when reporters are not explicitly provided by users.

Though I think it makes sense, this would potentially make it harder for this feature to reach some people. I still kinda want to see this feature to be enabled automatically for any condition on v1.3.0 and later consider tweaking conditions based on users feedback. So, if doing something is really necessary, then I might prefer introducing yet-another variable like VITEST_GITHUB_ACTIONS_REPORTER
#5093 (comment) to allow opting out. What do you think?

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