[ci][rust] Install nightly compiler for rust

As the builds now require the nightly compiler, install that before
trying to build with CI.

Signed-off-by: David Brown <david.brown@linaro.org>

Author: d3zd3z

.github/workflows/github-ci-rust.yml

@@ -69,6 +69,7 @@ jobs:
     - name: install rust
       shell: bash
       run: |
+        rustup install nightly
         rustup target add aarch64-unknown-linux-gnu
         rustup target add aarch64-unknown-none