jammy-amd64-tf-a-build: use rust stable toolchain

Change-Id: I6f9dc93ba0255fe1d48bf85e698051176c1732ce
Signed-off-by: Zachary Leaf <zachary.leaf@arm.com>
diff --git a/jammy-amd64-tf-a-build/Dockerfile b/jammy-amd64-tf-a-build/Dockerfile
index af17509..91e25c1 100644
--- a/jammy-amd64-tf-a-build/Dockerfile
+++ b/jammy-amd64-tf-a-build/Dockerfile
@@ -113,6 +113,7 @@
 RUN set -ex ;\
     cargo install 'tokei@12.1.*' ;\
     cargo install cargo-binutils ;\
+    rustup default stable ;\
     rustup target add aarch64-unknown-none ;\
     rustup component add llvm-tools rustfmt