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/ci.yml

@@ -42,12 +42,12 @@ jobs:
             libxkbcommon-dev libwayland-dev libx11-dev libxrandr-dev libxi-dev \
             libgl1-mesa-dev libgtk-3-dev
 
-      - uses: dtolnay/rust-toolchain@stable
+      - uses: dtolnay/rust-toolchain@b725eda525362e7cf34af9a7243cac1e3f01a7ec # stable
         with:
           components: clippy, rustfmt
 
       # Only main saves caches; PRs restore main's, so PR caches don't evict main's.
-      - uses: Swatinem/rust-cache@v2
+      - uses: Swatinem/rust-cache@d1527c8f67ef9af357290475d4af4b0fab8f12e0 # v2.9.3
         with:
           save-if: ${{ github.ref == 'refs/heads/main' }}
           cache-on-failure: true
@@ -75,10 +75,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v7
-      - uses: dtolnay/rust-toolchain@stable
+      - uses: dtolnay/rust-toolchain@b725eda525362e7cf34af9a7243cac1e3f01a7ec # stable
         with:
           targets: wasm32-unknown-unknown
-      - uses: Swatinem/rust-cache@v2
+      - uses: Swatinem/rust-cache@d1527c8f67ef9af357290475d4af4b0fab8f12e0 # v2.9.3
         with:
           key: wasm32
           save-if: ${{ github.ref == 'refs/heads/main' }}