Don't pass stdin to VMs unless necessary.

Fixes QubesOS/qubes-issues#5562

dom0-updates/qubes-dom0-update

@@ -201,7 +201,7 @@ qvm-run --nogui -q $UPDATEVM 'rm -rf /var/lib/qubes/dom0-updates/etc' || exit 1
 tar c /var/lib/rpm /etc/yum.repos.d /etc/yum.conf /etc/dnf/dnf.conf 2>/dev/null | \
    qvm-run --nogui -q --pass-io "$UPDATEVM" 'LC_MESSAGES=C tar x -C /var/lib/qubes/dom0-updates 2>&1 | grep -v -E "s in the future"'
 
-qvm-run $QVMRUN_OPTS --pass-io $UPDATEVM "script --quiet --return --command '/usr/lib/qubes/qubes-download-dom0-updates.sh --doit --nogui $(quote_args "${ALL_OPTS[@]}")' /dev/null"
+qvm-run $QVMRUN_OPTS --pass-io $UPDATEVM "script --quiet --return --command '/usr/lib/qubes/qubes-download-dom0-updates.sh --doit --nogui $(quote_args "${ALL_OPTS[@]}")' /dev/null" < /dev/null
 RETCODE=$?
 if [ "$CHECK_ONLY" == "1" ]; then
     exit $RETCODE