buildserver: switch to debian.sources

Since trixie debian.sources is recommended instead of sources.list. See

    https://wiki.debian.org/SourcesList

Fixes the following warning on apt-get update:

    W: Target Packages (main/binary-amd64/Packages) is configured multiple times in /etc/apt/sources.list:1 and /etc/apt/sources.list.d/debian.sources:1
    W: Target Packages (main/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:1 and /etc/apt/sources.list.d/debian.sources:1
    W: Target Packages (main/binary-amd64/Packages) is configured multiple times in /etc/apt/sources.list:3 and /etc/apt/sources.list.d/debian.sources:1
    W: Target Packages (main/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:3 and /etc/apt/sources.list.d/debian.sources:1
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