treewide: Update ECR public Rust images

This commit updates the Rust version of the public ECR library image we
use for our xtask crate and for our cargo-deny docker image builder. The
version is updated from 1.74 to 1.78.

Change-Id: Idad4f0c1727b5bc37c1f008281e450e8f63fa24d

build/Dockerfile.cargo-deny

@@ -1,3 +1,3 @@
-FROM 305232526136.dkr.ecr.us-east-2.amazonaws.com/ecr-public/docker/library/rust:1.74
+FROM 305232526136.dkr.ecr.us-east-2.amazonaws.com/ecr-public/docker/library/rust:1.78
 
 RUN cargo install cargo-deny --version 0.13.7 && cargo deny --version