[ci] Use specific rustup toolchain

Prevent surprise warnings from happening again in the future.

.github/workflows/rust-test.yml

@@ -32,6 +32,15 @@ jobs:
 
     steps:
       - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 #v4.1.1
+      - name: setup rust
+        run: |
+          # Specify an exact Rust version.
+          #
+          # Newer versions will enable newer features, but at the same time, you may get newer warnings which you have
+          # to fix. This should be a manual step.
+          rustup install 1.83.0
+          rustup default 1.83.0
+          rustup component add rustfmt clippy
       - name: sdk format
         working-directory: tuta-sdk/rust
         run: cargo fmt --check