website-build: check.sh: Improve grep usage.

Without that fix, using shellcheck on this file results in this
warning:
    In website-build/check.sh line 56:
    	nr_files=$(tar tf "${tarball}" | grep -v '/' | wc -l)
                                             ^---------^ SC2126 (style):
					     Consider using
					     'grep -c' instead of 'grep|wc -l'.

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>
Acked-by: Adrien 'neox' Bourmault <neox@gnu.org>

website-build/check.sh

@@ -53,7 +53,7 @@ test_savannah_cvs_constraints()
 	name="$1"
 	tarball="$2"
 
-	nr_files=$(tar tf "${tarball}" | grep -v '/' | wc -l)
+	nr_files=$(tar tf "${tarball}" | grep -c -v '/')
 
 	if [ "${nr_files}" = "1" ] ; then
 		echo "[ OK ] ${name}"