add env variable for parallel

[bertwagner]

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The LLaMA.cpp HTTP Server README mentions using environment variable LLAMA_ARG_N_PARALLEL for setting the value of --parallel. This environment variable is missing from common/arg.cpp and this PR adds it in.

[VJHack]

Nit: I would suggest that you just add (env: LLAMA_ARG_N_PARALLEL) to the docs here for clarity.

[ggerganov]

Let's merge after the updated help message as suggested

[bertwagner]

Thanks for the suggestion @VJHack . @ggerganov the README is now updated and ready to merge, thank you.