Fix 'access denied' error when running in restricted (read-only) env

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When container image deployed in read-only environment (e.g. restricted namespace), write-access is required for outputting "env.js" file. Redirecting output of this file to "/tmp" directory

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As an immediate fix we applied the patch on the level of helm charts: spiffe/helm-charts#326

This patch should be moved to the image and then removed from the helm charts

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Another attempt to fix it on the image level done by PR #253

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This issue un-blocks the helm-chart PR spiffe/helm-charts#328