kata:rust: Point the rust version to KATA_RUST_VERSION

So we don't need to maintain the version at two places.

Change-Id: Ibbbb9b7744bdd4e209404bf744e2f57e0022db7a
GitOrigin-RevId: 7583eaba9d5364e6c47ede3e5fed4505c73ae790

apps/system/rust.cmake

@@ -51,7 +51,7 @@ function(RustAddLibrary lib_name)
         WORKING_DIRECTORY ${RUST_SOURCE_DIR}
         COMMAND
             ${CMAKE_COMMAND} -E env RUSTFLAGS=${RUSTFLAGS}
-            cargo +nightly-2021-08-05 build
+            cargo "+$ENV{KATA_RUST_VERSION}" build
             --target ${RUST_TARGET}
             ${CARGO_OPTIONS} ${CARGO_RELEASE}
             --target-dir ${RUST_BUILD_DIR}