[SPARK-50527][INFRA][FOLLOW-UP] Python 3.12 image clean up

### What changes were proposed in this pull request?
Python 3.12 image clean up

### Why are the changes needed?
`python3.12` was installed twice

no similar issue were found in other new images

### Does this PR introduce _any_ user-facing change?
no, infra-only

### How was this patch tested?
CI, will monitor the Python 3.12 daily build

### Was this patch authored or co-authored using generative AI tooling?
no

Closes #49266 from zhengruifeng/infra_12_cleanup.

Authored-by: Ruifeng Zheng <ruifengz@apache.org>
Signed-off-by: Ruifeng Zheng <ruifengz@apache.org>

dev/spark-test-image/python-312/Dockerfile

@@ -71,13 +71,10 @@ ARG BASIC_PIP_PKGS="numpy pyarrow>=18.0.0 six==1.16.0 pandas==2.2.3 scipy plotly
 # Python deps for Spark Connect
 ARG CONNECT_PIP_PKGS="grpcio==1.67.0 grpcio-status==1.67.0 protobuf==5.29.1 googleapis-common-protos==1.65.0 graphviz==0.20.3"
 
-# Install Python 3.12 at the last stage to avoid breaking the existing Python installations
-RUN apt-get update && apt-get install -y \
-    python3.12 \
-    && rm -rf /var/lib/apt/lists/*
+# Install Python 3.12 packages
 RUN curl -sS https://bootstrap.pypa.io/get-pip.py | python3.12
 RUN python3.12 -m pip install --ignore-installed blinker>=1.6.2 # mlflow needs this
 RUN python3.12 -m pip install $BASIC_PIP_PKGS unittest-xml-reporting $CONNECT_PIP_PKGS lxml && \
     python3.12 -m pip install torch torchvision --index-url https://download.pytorch.org/whl/cpu && \
     python3.12 -m pip install torcheval && \
-    python3.12 -m pip cache purge \
+    python3.12 -m pip cache purge