Fix check-buildroot missing matches with grep >= 3.5

Since 3.5, grep emits the diagnostic "binary file matches" message
to stderr which causes the result file to be empty and build continuing
despite an obvious error being present.

We're not interested in the match itself, only whether there are files
with matches. Grep has a standard option for this (-l), use it.

Fixes: #1968

scripts/check-buildroot

@@ -29,7 +29,7 @@ NCPUS=${RPM_BUILD_NCPUS:-1}
 find "$RPM_BUILD_ROOT" \! \( \
  -name '*.pyo' -o -name '*.pyc' -o -name '*.elc' -o -name '.packlist' \
  \) -type f -print0 | \
- LANG=C xargs -0r -P$NCPUS -n16 grep -F "$RPM_BUILD_ROOT" >>$tmp
+ LANG=C xargs -0r -P$NCPUS -n16 grep -lF "$RPM_BUILD_ROOT" >>$tmp
 
 test -s "$tmp" && {
  cat "$tmp"