[CI] Fix failing arm build (#8751)

* Always install Conda env into /opt/python; use Mamba

* Change ownership of Conda env to buildkite-agent user

* Use unique name

* Fix

tests/ci_build/Dockerfile.rmm

@@ -13,8 +13,8 @@ RUN \
     apt-get update && \
     apt-get install -y wget unzip bzip2 libgomp1 build-essential ninja-build git && \
     # Python
-    wget -nv -O Miniconda3.sh https://repo.anaconda.com/miniconda/Miniconda3-latest-Linux-x86_64.sh && \
-    bash Miniconda3.sh -b -p /opt/python
+    wget -nv -O conda.sh https://github.com/conda-forge/miniforge/releases/download/22.11.1-2/Mambaforge-22.11.1-2-Linux-x86_64.sh && \
+    bash conda.sh -b -p /opt/mambaforge
 
 # NCCL2 (License: https://docs.nvidia.com/deeplearning/sdk/nccl-sla/index.html)
 RUN \
@@ -23,7 +23,7 @@ RUN \
     apt-get update && \
     apt-get install -y --allow-downgrades --allow-change-held-packages libnccl2=${NCCL_VERSION}+cuda${CUDA_SHORT} libnccl-dev=${NCCL_VERSION}+cuda${CUDA_SHORT}
 
-ENV PATH=/opt/python/bin:$PATH
+ENV PATH=/opt/mambaforge/bin:$PATH
 
 # Create new Conda environment with RMM
 RUN \