Ignore env vars with multiline values

[libesz]

Ignore env vars with multiline values. They are failing make targets if present.

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[istio-testing]

Hi @libesz. Thanks for your PR.

I'm waiting for a istio member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[libesz]

We've hit this issue now also in CI builds, not only locally. E.g. Travis sets up env vars like the actual commit message, which may be multiline.