Update Rust SDK build image (#2635)

Looks like some libraries that this depends on require a more up-to-date
Rust language release.

Unblocks CI.

build/build-sdk-images/rust/Dockerfile

@@ -22,7 +22,7 @@ RUN apt-get --allow-releaseinfo-change update && \
 ENV RUSTUP_HOME=/usr/local/rustup \
     CARGO_HOME=/usr/local/cargo \
     PATH=/usr/local/cargo/bin:$PATH \
-    RUST_VERSION=1.55.0
+    RUST_VERSION=1.61.0
 RUN wget -q https://static.rust-lang.org/rustup/dist/x86_64-unknown-linux-gnu/rustup-init && \
     chmod +x rustup-init && \
     ./rustup-init -y --no-modify-path --default-toolchain $RUST_VERSION && \