lib/output: forcibly disable dark background detection in Buildkite

Created by: bobheadxi

A customer reported:

So it looks like github.com/sourcegraph/sourcegraph/lib/output.NewOutput hangs when run by our Buildkite agent see line of code here I patched src-cli and use fmt.Print to isolate the problem to there. (I am unsure I can get a debugger here)

We ran into the same issue with sg usage in Buildkite, which we patched in https://github.com/sourcegraph/sourcegraph/pull/36193. We can't easily apply a similar patch for src because there are many points where output is initialized (query) but at the end of the day I think it makes sense for detectCapabilities to house this logic, since all users of this package will run into this issue and it fits in with the need to "detect capabilities"

Test plan

n/a