chore: Update Debian before installing git-core
chore: Update Debian before installing git-core

When the package database becomes out of date, it can have references to
versions that have been removed from the debian package mirror which
causes the build to fail.

    E: Failed to fetch http://security-cdn.debian.org/debian-security/pool/updates/main/g/git/git_2.11.0-3+deb9u3_amd64.deb  404  Not Found
    E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?

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