shellcheck: work around old versions of shellcheck

... that do not support `--format=json1`.

Related: https://issues.redhat.com/browse/OSH-655

scripts/run-shellcheck.sh

@@ -43,8 +43,16 @@ filter_shell_scripts"' "$@"'
 # function that creates a separate JSON file if shellcheck detects anything
 wrap_shellcheck() {
     dst="${SC_RESULTS_DIR}/sc-$$.json"
-    (set -x && timeout ${SC_TIMEOUT} shellcheck --format=json1 "$@" > "$dst")
+
+    # This is a compatibility workaround for old versions of shellcheck
+    # that do not support `--format=json1`.
+    echo '{"comments":' > "$dst"
+
+    (set -x && timeout ${SC_TIMEOUT} shellcheck --format=json "$@" >> "$dst")
     EC=$?
+
+    echo '}' >> "$dst"
+
     case $EC in
         0)
             # no findings detected -> remove the output file