rootbld: allow the fake $HOME to be writable

this is needed for running some test runners such as kyua under rootbld.

abuild.in

@@ -2159,6 +2159,7 @@ rootbld() {
 		--ro-bind "$BUILD_ROOT" / \
 		--proc /proc \
 		--dev-bind /dev /dev \
+		--bind "$BUILD_ROOT/$HOME" "$HOME" \
 		--ro-bind "$HOME/.abuild" "$HOME/.abuild" \
 		--ro-bind "$aportsgit" "$aportsgit" \
 		--bind "$SRCDEST" "$SRCDEST" \