Fix let_underscore_drop lint error in dice_tracker

[daandemeyer]

OSS CI (test.py / cargo clippy) is currently failing on every PR and on main itself with:

error: non-binding let on a type that has a destructor
   --> app/buck2_server/src/dice_tracker.rs:138:9

0578509 added the HydrationFailed variant with a String field to DiceEvent, giving it a destructor. That armed the deny-listed let_underscore_drop lint (lint_levels.bzl) on the pre-existing let _ = in BuckDiceTracker::event. Bind to _ignored instead, matching other call sites that discard unbounded_send results.

🤖 Generated with Claude Code

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D113539998. (Because this pull request was imported automatically, there will not be any future comments.)

[ndmitchell]

Review automatically exported from Phabricator review in Meta.

[daandemeyer]

The macos failure here is an unrelated timing flake: epoch::worker::tests::test_cancellation_sync_correct_context_does_not_cache panicked with elapsed_ms=650 but expected < 650 — exactly on the boundary on a slow runner. The same test passed on linux and windows in this run, and the identical two commits went green on all three platforms (macos included) in the CI run for #1364, which stacks on top of this fix.

[meta-codesync]

This pull request has been merged in 3c9f680.

[ndmitchell]

Thanks!