jammy-amd64-tf-a-build: Use explicit Rust version; update to 1.82.0

We also install the needed version in 1st invocation of the downloaded
rustup script, and avoid 2nd call to "rustup default".

Addresses https://linaro.atlassian.net/browse/TFC-677

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I638c2fd695e96d85950d48b4e858ce9fef40a3ac
diff --git a/jammy-amd64-tf-a-build/Dockerfile b/jammy-amd64-tf-a-build/Dockerfile
index 91e25c1..c02eb61 100644
--- a/jammy-amd64-tf-a-build/Dockerfile
+++ b/jammy-amd64-tf-a-build/Dockerfile
@@ -109,11 +109,10 @@
     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 1.82.0 -y
 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