Pin future-tasks version to 0.3.29

Workaround https://github.com/rust-lang/rust/issues/32976 from 0.3.30

Change-Id: Icacc7f46deb9c669327afdd3a3402186e53c7112

config/Cargo.toml

diff --git a/config/Cargo.toml b/config/Cargo.toml
index a9bff0d..2d724ac 100644
--- a/config/Cargo.toml
+++ b/config/Cargo.toml
@@ -2,3 +2,6 @@
 name = "matcha_config"
 version = "0.1.0"
 edition = "2018"
+
+[dependencies]
+futures-task = { version = "=0.3.29", default-features = false, features = ["unstable", "cfg-target-has-atomic"] }