Add env var QDKTELTEST_TELEMETRY_OPT_OUT

[emanuelb]

https://github.com/microsoft/qsharp-compiler/blob/96258b0a5e7fa13f711ebfa05bb190e83aca5c8c/src/Telemetry/Tests/TelemetryManagerTests.cs#L20

Related issue is #461 (same repo)

Maybe this env affect only tests and not the product itself? as it's under Tests directory.