ci: pin GitHub Actions to full commit SHA (SonarCloud githubactions:S7637)

Pin third-party actions to full-length commit SHAs (with version comments)
instead of mutable tags/branches to prevent supply-chain tampering via
retagged releases. Resolves SonarCloud rule githubactions:S7637.

.github/workflows/printing.yml

@@ -50,7 +50,7 @@ jobs:
             pkg-config libxkbcommon-dev libwayland-dev libx11-dev libxcursor-dev libxrandr-dev \
             libxi-dev libgtk-3-dev libfontconfig1-dev libfreetype-dev libvulkan1 mesa-vulkan-drivers
 
-      - uses: dtolnay/rust-toolchain@stable
+      - uses: dtolnay/rust-toolchain@b725eda525362e7cf34af9a7243cac1e3f01a7ec # stable
         with:
           components: rustfmt, clippy
           targets: wasm32-unknown-unknown