[rust, sound] Lock base64ct version to 1.6.0

1.7.1 is not compatible with the cached rust toolchain

Change-Id: I92afb5f9da3bd9063295a144d572af5014dff1d4

apps/rust/sound/Cargo.toml

diff --git a/apps/rust/sound/Cargo.toml b/apps/rust/sound/Cargo.toml
index e6ffc36..14d6167 100644
--- a/apps/rust/sound/Cargo.toml
+++ b/apps/rust/sound/Cargo.toml
@@ -23,7 +23,7 @@
 crate-type = ["staticlib"]
 
 [dependencies]
-base64ct = { version = "1.6",  default-features = false, features = ["alloc"] }
+base64ct = { version = "=1.6.0",  default-features = false, features = ["alloc"] }
 cantrip-os-common = { path = "../../system/components/cantrip-os-common", default-features = false }
 libcantrip = { path = "../libcantrip" }
 log = "0.4"