Output is truncated heavily in ARM64 when a command hangs

In https://travis-ci.org/github/native-api/test_travis/jobs/663728118, I accidentally read stdin with cat -n "${FF[@]}" when $FF happened to be empty.

It caused the build to hang (understandably), but output got truncated mid-line and long before the offending code.
(A sibling job in AMD64 environment, with exact same settings, correctly displayed all output up to the offending line.)

In another build with this defect, https://travis-ci.org/github/native-api/test_travis/jobs/663783018, the output got truncated at exactly the same place – which suggests that the output defect is connected to stream buffering.

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The stream buffering issue also seems to cause timeouts for Arm64 jobs.

And since the log is incomplete (due to buffering) it looks like the job hung in the middle of an earlier command, sending the customer down a false trail.