Use cmake from normal repo, if no backports

If installing from backports does not work, install cmake
normally. This might be too old, but we handle that anyway.
But on a much newer debian, it might be good enough.

legacy/setup-debian.sh

@@ -10,5 +10,6 @@ hostname libbz2-dev libcurl4-openssl-dev libicu-dev libgl1-mesa-dev libglu1-mesa
 libncurses-dev libssl-dev libtool libx11-dev libxerces-c-dev libxext-dev libxft-dev \
 libxml2-dev libxmu-dev libxpm-dev libyaml-cpp-dev lsb-release make patch python-dev \
 python3-dev rsync sed subversion tar unzip wget xutils-dev xz-utils
-apt-get -t buster-backports install cmake
+apt-get -y -t buster-backports install cmake || \
+	apt-get -y install cmake
 apt-get -y clean