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GH-14993: [CI][Conda] Fix missing RECIPE_ROOT variable now expected by conda build (#15014)

Fixes #14993


* Closes: #14993

Lead-authored-by: H. Vetinari <h.vetinari@gmx.com>
Co-authored-by: Sutou Kouhei <kou@clear-code.com>
Co-authored-by: Uwe L. Korn <uwe.korn@quantco.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>