jammy-amd64-tf-a-build-next: Support Nightly Rust

Also install the Rust nightly toolchain to allow testing of unstable
features like BTI and PAC.

Change-Id: Ibda04f4d75de5688cd116c71aff8d1386210929d
Signed-off-by: Matthew Maurer <mmaurer@google.com>
diff --git a/jammy-amd64-tf-a-build-next/Dockerfile b/jammy-amd64-tf-a-build-next/Dockerfile
index c0de9ce..de4647a 100644
--- a/jammy-amd64-tf-a-build-next/Dockerfile
+++ b/jammy-amd64-tf-a-build-next/Dockerfile
@@ -109,7 +109,8 @@
     rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
 
 RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | \
-        sh -s -- --default-toolchain stable -y
+        sh -s -- --default-toolchain stable -y && \
+    rustup install nightly
 RUN cargo install 'tokei@12.1.*'
 
 ENV NVM_DIR="/home/buildslave/.nvm"