ci: pin cmake 3.31 in ubuntu 20.04 earthly image

Author: Knogle

docker/ci/linux-build.Dockerfile

@@ -9,12 +9,14 @@ RUN dpkg --add-architecture i386 \
         ca-certificates \
         git \
         cmake \
+        python3-pip \
         build-essential \
         pkg-config \
         binutils \
         gcc-multilib \
         g++-multilib \
         libssl-dev:i386 \
         tar \
+    && pip3 install --no-cache-dir "cmake==3.31.6" \
+    && cmake --version \
     && rm -rf /var/lib/apt/lists/*
-