Gitlab CI: Test cmake build on Debian

Was seeing ModuleNotFoundError: No module named 'skbuild' when using
pip3 to install cmake on Ubuntu 16.  Installing with apt-get may resolve
the issue, and switching to use debian:testing to be certain a newer
version of cmake is available.

.gitlab-ci.yml

@@ -47,13 +47,12 @@ debian:testing:
     - apt-get update -y
     - apt-get install -y gcc meson python3-distutils
 
-ubuntu:bionic:cmake:
+debian:testing:cmake:
   extends: '.cmake_test'
-  image: $AMD64_UBUNTU_BIONIC
+  image: $AMD64_DEBIAN_TESTING
   before_script:
     - apt-get update -y
-    - apt-get install -y gcc make python3-pip
-    - pip3 install cmake
+    - apt-get install -y gcc make python3-pip cmake
 
 ubuntu:bionic:meson:
   extends: '.meson_test'