Fix incorrect hard-coded env var name in Java tracer configurator

[nikita-tkachenko-datadog]

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What does this PR do?

Fixes an error in Java tracer configurator code: when retrieving a config value from environment variable it was using an incorrect hard-coded variable name.
The error was not caught by the existing integration tests, as only manifested itself when that particular env variable was set (that code uses variables to override default tracer configuration settings).

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