abuild-sudo: also look for adduser/group in /usr/sbin

seems like it moved in recent busybox. we need to support both variants

abuild-sudo.c

@@ -14,7 +14,9 @@
 
 static const char* valid_cmds[] = {
 	"/bin/adduser",
+	"/usr/sbin/adduser",
 	"/bin/addgroup",
+	"/usr/sbin/addgroup",
 	"/sbin/apk",
 	NULL
 };
@@ -24,6 +26,8 @@ const char *get_command_path(const char *cmd)
 	const char *p;
 	int i;
 	for (i = 0; valid_cmds[i] != NULL; i++) {
+		if (access(valid_cmds[i], F_OK) == -1)
+			continue;
 		p = strrchr(valid_cmds[i], '/') + 1;
 		if (strcmp(p, cmd) == 0)
 			return valid_cmds[i];