thanos: add meta.mainProgram

Changed files
+1
pkgs
servers
monitoring
thanos
+1
pkgs/servers/monitoring/thanos/default.nix
···
homepage = "https://github.com/thanos-io/thanos";
changelog = "https://github.com/thanos-io/thanos/releases/tag/v${version}";
license = licenses.asl20;
+
mainProgram = "thanos";
maintainers = with maintainers; [ basvandijk anthonyroussel ];
};
}