DAOS-16875 cq: fix flake8 xargs usage (#15608)

Use -r so if no scons or non-scons files are grep'ed, flake8 does not
run.

Signed-off-by: Dalton Bohning <[email protected]>

utils/githooks/pre-commit.d/71-flake.sh

@@ -48,12 +48,12 @@ else
     rc=0
 
     # non-scons
-    if ! echo "$py_files" | grep -vi scons | xargs flake8 --config .flake8; then
+    if ! echo "$py_files" | grep -vi scons | xargs -r flake8 --config .flake8; then
         rc=1
     fi
 
     # scons
-    if ! echo "$py_files" | grep -i scons | xargs flake8 --config .flake8-scons; then
+    if ! echo "$py_files" | grep -i scons | xargs -r flake8 --config .flake8-scons; then
         rc=1;
     fi