ci: quarantine walk_dir stall-budget flake under the ci profile (rustfs#4690)
First application of the flake policy from docs/testing/README.md: the
test failed on a zero-Rust-diff PR (#4674, run 29099382470) — timing
windows in the stall-budget accounting stretch under CI load. retries=2
under profile.ci only; local default profile still never retries.
Tracked by rustfs#4690 (30-day fix-or-delete deadline).