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ARROW-14932: [CI] Prefer mamba over conda

Mamba should provide quicker image builds compared to conda.

Closes #11816 from kszucs/mamba

Authored-by: Krisztián Szűcs <szucs.krisztian@gmail.com>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>