spread tests fail on non-interactive prompt in ruby/legacy-pull

[anatoly techtonik]

https://travis-ci.org/snapcore/snapcraft/jobs/552155933

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2019-06-29 14:55:24 WARNING: jun291409-049240 (google:ubuntu-18.04-64:tests/spread/plugins/ruby/legacy-pull) running late. Current output:
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(...)
Restart services during package upgrades without asking? [yes/no]
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.
The job exceeded the maximum time limit for jobs, and has been terminated.

Looks like the message is shows when a package installation requires configuration https://unix.stackexchange.com/questions/146283/how-to-prevent-prompt-that-ask-to-restart-services-when-installing-libpq-dev The solution is to add `export DEBIAN_FRONTEND=noninteractive` to apt install command, but it is not clear where these command are.

Failing legacy-pull task doesn't contain and `apt` commands https://github.com/snapcore/snapcraft/blob/master/tests/spread/plugins/ruby/legacy-pull/task.yaml and main `spread.yaml` is shared between tests, but they do not fail. So the problem should be in legacy `snapcraft pull` that probably fetches packages in interactive mode.