GH-40112: [CI][Python] Ensure CPython is selected, not PyPy (#40131)

Sometimes, mamba might select PyPy over CPython in certain environment upgrade/downgrade scenarios.

* Closes: #40112

Lead-authored-by: Antoine Pitrou <antoine@python.org>
Co-authored-by: Uwe L. Korn <uwe.korn@quantco.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>

ci/docker/conda-python.dockerfile

@@ -28,7 +28,7 @@ COPY ci/conda_env_python.txt \
 RUN mamba install -q -y \
         --file arrow/ci/conda_env_python.txt \
         $([ "$python" == $(gdb --batch --eval-command 'python import sys; print(f"{sys.version_info.major}.{sys.version_info.minor}")') ] && echo "gdb") \
-        python=${python} \
+        "python=${python}.*=*_cpython" \
         nomkl && \
     mamba clean --all