mkarchiso: redirect command -v output to /dev/null

The output is irrelevant, we only need the return code.

Related to #148.

archiso/mkarchiso

@@ -748,7 +748,7 @@ _validate_requirements_airootfs_image_type_ext4+squashfs() {
 }
 
 _validate_requirements_airootfs_image_type_erofs() {
-    if ! command -v mkfs.erofs; then
+    if ! command -v mkfs.erofs &> /dev/null; then
         (( validation_error=validation_error+1 ))
         _msg_error "Validating '${airootfs_image_type}': mkfs.erofs is not available on this host. Install 'erofs-utils'!" 0
     fi