Fixed 0.4.15.5 Debian package

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The issue with the packaging of Neubot for Debian has been resolved by releasing 0.4.15.5-2. The problem was the conditional assignment of the USER variable in the Makefile, unless the variable was set in the environment. Since USER is of course always set on UNIX, I had rolled out by mistake Debian packages with unprivileged user equal to "simone". Fixed by this commit: https://github.com/neubot/neubot/commit/ce642998