repart: Create disk image file with copy-on-write disabled on btrfs

COW on btrfs generally does not play well lots of random writes so
let's make the disk images generated by repart NOCOW by default on
btrfs like we do elsewhere across the codebase.

src/partition/repart.c

@@ -7909,7 +7909,7 @@ static int find_root(Context *context) {
                         if (!s)
                                 return log_oom();
 
-                        fd = open(arg_node, O_RDONLY|O_CREAT|O_EXCL|O_CLOEXEC|O_NOFOLLOW, 0666);
+                        fd = xopenat_full(AT_FDCWD, arg_node, O_RDONLY|O_CREAT|O_EXCL|O_CLOEXEC|O_NOFOLLOW, XO_NOCOW, 0666);
                         if (fd < 0)
                                 return log_error_errno(errno, "Failed to create '%s': %m", arg_node);