fix build of VirtualBox 6.1.8

help-steps/cowbuilder_build_virtualbox_chroot_script

@@ -34,6 +34,20 @@ done
       -o Dir::Etc::sourcelist="/tmp/temporary-repository/newer.list" \
       -o Dir::Etc::sourceparts="-"
 
+## The 'apt-get build-dep' command below would cause the following error message:
+##
+# Some packages could not be installed. This may mean that you have
+# requested an impossible situation or if you are using the unstable
+# distribution that some required packages have not yet been created
+# or been moved out of Incoming.
+# The following information may help to resolve the situation:
+#
+# The following packages have unmet dependencies:
+#  libc6-dev : Breaks: libgcc-8-dev (< 8.4.0-2~) but 8.3.0-6 is to be installed
+##
+## Therefore purging package 'libgcc-8-dev' to make next 'apt-get build-dep' command work.
+apt-get purge --yes libgcc-8-dev
+
 apt-get \
    build-dep \
       $APTGETOPT_ALT \