[Build] Fix dbus-python installation issue in armhf (#12849)

Why I did it
Fix the python3 dbus-python installation issue in arm.

See https://dev.azure.com/mssonic/be1b070f-be15-4154-aade-b1d3bfb17054/_apis/build/builds/180980/logs/96

      
            CMake Error at CMakeLists.txt:1 (cmake_minimum_required):
              CMake 3.15 or higher is required.  You are running version 3.13.4
How I did it
Install the cmake package from buster-backports

How to verify it
Succeeded:

docker run --rm -it 8380970b72ea bash
sudo apt-get install -y -t buster-backports cmake
sudo pip3 install dbus-python
Where the docker image 8380970b72ea is the buster slave image in armhf.

Failed:

docker run --rm -it 8380970b72ea bash
sudo pip3 install dbus-python

sonic-slave-buster/Dockerfile.j2

@@ -354,6 +354,9 @@ RUN apt-get update && apt-get install -y \
         libsystemd-dev \
         pkg-config
 
+# Upgrade cmake to support dbus-python build in armhf
+RUN apt-get install -t buster-backports -y cmake
+
 RUN apt-get -y build-dep openssh
 
 # Build fix for ARMHF buster libsairedis