modify: make it pass shellcheck

Without that fix, running 'shellcheck -x build' produces the following
errors/warnings:
    In modify line 23:
    [ "x${DEBUG+set}" = 'xset' ] && set -v
      ^-------------^ SC2268 (style):
      Avoid x-prefix in comparisons as it no longer serves a purpose.

    In modify line 106:
    resources/packages/"${package}"/modify/"${option}" $@
                                                       ^-- SC2068 (error):
    Double quote array expansions to avoid re-splitting elements.

    In modify line 113:
    "${pkg_dir}"/modify/"${option}" $@
                                    ^-- SC2068 (error):
    Double quote array expansions to avoid re-splitting elements.

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>
Acked-by: Adrien 'neox' Bourmault <neox@gnu.org>

modify

@@ -20,7 +20,7 @@
 #    along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #
 
-[ "x${DEBUG+set}" = 'xset' ] && set -v
+[ "${DEBUG+set}" = 'set' ] && set -v
 set -u -e
 
 projectname="$(cat projectname)"
@@ -103,6 +103,7 @@ if [ $# -gt 1 ]; then
 		;;
 	all)
 		for option in $(listoptions "${package}"); do
+			# shellcheck disable=SC2068
 			resources/packages/"${package}"/modify/"${option}" $@
 		done
 		;;
@@ -110,6 +111,7 @@ if [ $# -gt 1 ]; then
 		if [ -d resources/packages/"${package}"/modify ]; then
 			pkg_dir=resources/packages/"${package}"
 			if [ -f "${pkg_dir}"/modify/"${option}" ]; then
+				# shellcheck disable=SC2068
 				"${pkg_dir}"/modify/"${option}" $@
 			else
 				help

tests/lint

@@ -42,6 +42,7 @@ printf "| Running lint tests: |\n"
 printf "+---------------------+\n"
 run_shellcheck \
     build \
+    modify \
     resources/packages/descriptors/distclean \
     resources/packages/flashrom/distclean \
     resources/packages/grub/distclean \