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ARROW-5919: [R] Test R-in-conda as a nightly build

Author: Uwe L. Korn <uwe.korn@quantco.com>
Author: Uwe L. Korn <xhochy@users.noreply.github.com>

Closes #4855 from xhochy/conda-r-docker and squashes the following commits:

b3189b863 <Uwe L. Korn> Add LICENSE to conda_env_r
971181dec <Uwe L. Korn> Add to nightlies
743aa5955 <Uwe L. Korn> Update Dockerfile.conda
b79e0c08e <Uwe L. Korn> ARROW-XXXX:  Test R-in-conda as a nightly build