agent(rhel8): call grep with -q

As we don't care about the matched output, only that it matched.

Follow-up for 7a37f0b.

Author: mrc0mmand

agent/testsuite-rhel8.sh

@@ -155,7 +155,7 @@ for t in test/TEST-??-*; do
     # about them, so let's tell it to whitelist all known syscalls
     export NSPAWN_ARGUMENTS="--system-call-filter=@known"
 
-    if grep 'seccomp_init_for_arch(.*SCMP_ACT_ERRNO(EPERM)' src/nspawn/nspawn-seccomp.c; then
+    if grep -q 'seccomp_init_for_arch(.*SCMP_ACT_ERRNO(EPERM)' src/nspawn/nspawn-seccomp.c; then
         # In very old RHEL 8 systemd versions (8.6.0 and older) we're missing patch series [0] that configure
         # nspawn's seccomp filters to return ENOSYS instead of EPERM for unknown syscalls. And given we have
         # to run the CI for RHEL 8 systemd on C9S, we get a bunch of unknown syscalls when running nspawn which