chore: test flake: TestCoderLog

[johnstcn]

--- FAIL: TestCoderLog (0.00s)
    --- FAIL: TestCoderLog/OutputNotDropped (0.01s)
        coder_test.go:100: 
            	Error Trace:	/home/runner/work/envbox/envbox/buildlog/coder_test.go:100
            	Error:      	Condition never satisfied
            	Test:       	TestCoderLog/OutputNotDropped
    --- FAIL: TestCoderLog/OK (0.01s)
        coder_test.go:51: 
            	Error Trace:	/home/runner/work/envbox/envbox/buildlog/coder_test.go:51
            	Error:      	Condition never satisfied
            	Test:       	TestCoderLog/OK
FAIL

I could only get it to reproduce somewhat reliably by running

go test -count=10000 ./buildlog/

Interestingly, the race detector didn't seem to pick anything up.