appveyor: add support for using custom CMake versions

To allow more flexibility and not be limited by defaults offered by
the runner machines:
- Visual Studio 2013: CMake 3.12.2
- Visual Studio 2015, 2017: CMake 3.16.2
Ref: https://www.appveyor.com/docs/windows-images-software/

Start using 3.18.4, 3.19.8, 3.20.6 in older VS jobs to add variations.

Time cost is a couple of seconds per job.

Ref: #18704 (Discussion)
Ref: #16973

Closes #19737

appveyor.sh

@@ -42,6 +42,23 @@ elif [ "${APPVEYOR_BUILD_WORKER_IMAGE}" = 'Visual Studio 2019' ]; then
 fi
 
 if [ "${BUILD_SYSTEM}" = 'CMake' ]; then
+  # Install custom cmake version
+  if [ -n "${CMAKE_VERSION:-}" ]; then
+    cmake_ver=$(printf '%02d%02d' \
+      "$(echo "$CMAKE_VERSION" | cut -f1 -d.)" \
+      "$(echo "$CMAKE_VERSION" | cut -f2 -d.)")
+    if [ "${cmake_ver}" -ge '0320' ]; then
+      fn="cmake-${CMAKE_VERSION}-windows-x86_64"
+    else
+      fn="cmake-${CMAKE_VERSION}-win64-x64"
+    fi
+    curl --disable --fail --silent --show-error --connect-timeout 15 --max-time 60 --retry 3 --retry-connrefused \
+      --location "https://github.com/Kitware/CMake/releases/download/v${CMAKE_VERSION}/${fn}.zip" --output bin.zip
+    7z x -y bin.zip >/dev/null
+    rm -f bin.zip
+    PATH="$PWD/${fn}/bin:$PATH"
+  fi
+
   # Set env CHKPREFILL to the value '_chkprefill' to compare feature detection
   # results with and without the pre-fill feature. They have to match.
   for _chkprefill in '' ${CHKPREFILL:-}; do