lib/output: forcibly disable dark background detection in Buildkite

[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 #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

[jlisee]

I did a quick deep dive (using sourcegraph itself to walk the cross repo call chain), and found the smoking gun and root fix - muesli/termenv#52 - Don't run OSC queries on CI

Updating muesli/termenv to >= 0.11 in sourcegraph/src-cli also fixes the issue without needing any code changes or Buildkite knowledge.

For reference the call chain - The call chain:

• sourcegraph/sourcegraph/lib/output - NewOutput
• muesli/termenv HasDarkBackground -> isTTY
• mattn/go-isatty IsTerminal

[bobheadxi]

Thank you @jlisee for digging deeper :) I'll make the appropriate updates!