lbmk: use x_ instead of err, where appropriate

many places in lbmk used err, because older versions
of x_ did not handle globbing properly.

however, use of x_ is preferable on trivial commands.

the only time err() should be called is what it has
to be, when x_ can't work, or when a more useful error
message is needed, for context.

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/lib.sh

@@ -58,7 +58,7 @@ xbmkget()
 	for url in "$1" "$2"; do
 		[ "$dl_fail" = "n" ] && break
 		[ -z "$url" ] && continue
-		rm -f "$cached" || err "!rm -f '$cached'"
+		x_ rm -f "$cached"
 		if [ "$_dlop" = "curl" ]; then
 			curl --location --retry 3 -A "$_ua" "$url" \
 			    -o "$cached" || wget --tries 3 -U "$_ua" "$url" \