Update CMake to 3.15 (required by ninja)

Author: Kobzol

src/ci/docker/host-x86_64/dist-x86_64-linux/Dockerfile

@@ -88,7 +88,7 @@ COPY host-x86_64/dist-x86_64-linux/build-python.sh /tmp/
 # Build Python 3 needed for LLVM 12.
 RUN ./build-python.sh 3.9.1
 
-# LLVM needs cmake 3.13.4 or higher.
+# Ninja needs cmake 3.15 or higher.
 COPY host-x86_64/dist-x86_64-linux/build-cmake.sh /tmp/
 RUN ./build-cmake.sh
 

src/ci/docker/host-x86_64/dist-x86_64-linux/build-cmake.sh

@@ -3,7 +3,7 @@
 set -ex
 source shared.sh
 
-CMAKE=3.13.4
+CMAKE=3.15.6
 curl -L https://github.com/Kitware/CMake/releases/download/v$CMAKE/cmake-$CMAKE.tar.gz | tar xzf -
 
 mkdir cmake-build