workload-env-default

[jjtischer]

What this PR does / why we need it:
Ability to customize the WORKLOAD_IDENTITY ENV variable
This will make using RBE easier for building proxy on a different google cloud project

[google-cla]

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[istio-policy-bot]

😊 Welcome @jjtischer! This is either your first contribution to the Istio proxy repo, or it's been
awhile since you've been here.

You can learn more about the Istio working groups, code of conduct, and contributing guidelines
by referring to Contributing to Istio.

Thanks for contributing!

Courtesy of your friendly welcome wagon.

[istio-testing]

Hi @jjtischer. 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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