Skip test_highly_nested_objects_decoding during the PGO profile task. (GH-151460)

Skip test_highly_nested_objects_decoding during the PGO profile task. (GH-151460)

Since the recursion guard tracks real C-stack bounds (gh-91079), this test
asserts that 500k nesting levels overflow the stack margin. On a 64 MiB stack
(some Nix build envs use one that large), the optimized interpreter uses ~160
bytes/level (raises at ~420k levels) so the assertion holds with only ~16%
margin; the PGO *instrumented* stage inlines less, its per-level scanner frames
are smaller, and the 500k-deep decode completes -- "RecursionError not raised"
fails the profile run and aborts `make profile-opt`. Upstream's
skip_if_unlimited_stack_size (gh-143460) only covers RLIM_INFINITY, not
large-finite stacks like ours.

We could also keep playing whack a mole and raise the 500k to a much larger
number... but there's little value in PGO training on this test anyways.