fixed "in place upgrade issue - base-passwd debconf interative question asks 'Remove group "qubes"'"

(reported by @adrelanos)
fixed by no longer using static gid (fix suggested by @marmarek)
https://github.com/QubesOS/qubes-issues/issues/1105

debian/qubes-core-agent.preinst

@@ -37,7 +37,7 @@ if [ "$1" = "install" ] ; then
     # --------------------------------------------------------------------------
     # Required groups
     # --------------------------------------------------------------------------
-    groupadd --force --system --gid 98 qubes
+    groupadd --force --system qubes
     groupadd --force --system sudo
 
     # --------------------------------------------------------------------------