Only print fancy startup lines in interactive session

[fikovnik]

This is how it looks when stdout sinks to a file

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<U+2022> R           4.0.2 (2020-06-22)
<U+2022> instrumentr 0.1 (837debb)
<U+2022> injectr     0.1
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<U+2022> srcref      1056 base srcrefs added in 0.300000 seconds
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