fix(sampling): update sample rate env var in parametric test

[genesor]

Motivation

One of the parametric test is using an env var that is not used anymore by the python client. DD_TRACE_SAMPLE_RATE has been dropped and replaced by DD_TRACE_SAMPLING_RULES

The test is currently passing due to an hardcoded sampling decision USER_KEEP in the http header extractor that is being reworked dd-trace-py#12950

Changes

• add DD_TRACE_SAMPLING_RULES env var to Test_Decisionless_Extraction

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