Flaky integration tests

Integration tests fail sporadically

```
go test -tags integration ./integration/... -count=10
--- FAIL: Test_ConcurrentBuilds (2.85s)
    integration_test.go:161: concurrent builds running k6 fork/exec /tmp/Test_ConcurrentBuilds1548263293/002/k6: text file busy
FAIL
FAIL    github.com/grafana/k6build/integration  27.153s
FAIL
```