prevent broken junit report files

[xrstf]

This code was missing O_TRUNC.

Without it, my first hydrophone run randomly had issues and was dumping a 142MB junit_01.xml, containing lots of pod logs. I then re-ran hydrophone on another cluster where the tests succeeded, but I still seemingly got a 142MB file.

In reality, hydrophone was fetching a 2.5MB file and was only updating/replacing the first 2.5MB of my junit_01.xml file, instead of overwriting it completely.

Adding O_TRUNC would make this more or less equivalent to OpenFile(), except that Create() sets file permissions to 0666 instead of our 0600, which I think is perfectly acceptable. The junit/e2e logs are not secret data that should be kept from other users on the system.

[k8s-ci-robot]

Hi @xrstf. Thanks for your PR.

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[xmudrii]

/ok-to-test

[dims]

/approve
/lgtm

[k8s-ci-robot]

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