firstboot: change /etc/shadow access mode to 000

It appears to be customary to remove all access bits from /etc/shadow
including those for the root owner), hence let's do the same.

src/firstboot/firstboot.c

@@ -531,6 +531,7 @@ static int write_root_shadow(const char *path, const struct spwd *p) {
         assert(p);
 
         mkdir_parents(path, 0755);
+        RUN_WITH_UMASK(0777)
                 f = fopen(path, "wex");
         if (!f)
                 return -errno;