fix: Preserve AWS_CONTAINER_CREDENTIALS_RELATIVE_URI in nginx.conf

[fmtng]

"AWS_CONTAINER_CREDENTIALS_RELATIVE_URI" is set when "00-check-for-required-env.sh" runs. In "awscredentials.js" -> "fetchCredentials", however, the "AWS_CONTAINER_CREDENTIALS_RELATIVE_URI" is not set anymore and erroneously, "_fetchEC2RoleCredentials" instead of "_fetchEcsRoleCredentials" is called.

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[github-actions]

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[fmtng]

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[dekobon]

Thank you for the contribution!