Lock futures crate to in-tree v0.3.29.

BUG=376047660

Change-Id: If01c75868cc0019b0565be5a8f877822ebe019c2

config/Cargo.toml

diff --git a/config/Cargo.toml b/config/Cargo.toml
index 2d724ac..31f7af2 100644
--- a/config/Cargo.toml
+++ b/config/Cargo.toml
@@ -4,4 +4,5 @@
 edition = "2018"
 
 [dependencies]
-futures-task = { version = "=0.3.29", default-features = false, features = ["unstable", "cfg-target-has-atomic"] }
+# Locked to yanked futures crate version; see BUG(376047660).
+futures-task = { path = "../../libtock-rs/crates/futures-rs-0.3.29/futures-task", default-features = false, features = ["unstable", "cfg-target-has-atomic"] }