Set PYTHONHASHSEED for umitools dedup

[SPPearce]

The umi_tools dedup command is not currently deterministic, even if --random-seed is set, unless the PYTHONHASHSEED environment variable is also set. Detailed in CGATOxford/UMI-tools#440 and https://github.com/CGATOxford/UMI-tools/pull/470. Not fixed in the main umitools, so just explicitly export the environment variable here.

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