CI: Enable rust caching for Windows, if vcpkg dependencies unchanged.

Our non-Rust dependency versions are fixed on Windows by our choice of
vcpkg commit ID, so by using this as a key to the rust-cache action, we
can cache safely.

.github/workflows/rust.yml

@@ -44,11 +44,17 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     env:
+      VCPKG_COMMIT: 01f602195983451bc83e72f4214af2cbc495aa94 # 2024.05.24 release
       VCPKG_INSTALLED_DIR: ${{ github.workspace }}/vcpkg/installed
 
     steps:
       - uses: actions/checkout@v4
 
+      - uses: Swatinem/rust-cache@v2
+        with:
+          env-vars: VCPKG_COMMIT
+        if: runner.os == 'Windows'
+
       - name: Install toolchain
         run: |
           rustup toolchain install ${{ matrix.rust }}
@@ -79,7 +85,7 @@ jobs:
       - name: Install dependencies (Windows)
         uses: lukka/run-vcpkg@v11
         with:
-          vcpkgGitCommitId: 01f602195983451bc83e72f4214af2cbc495aa94 # 2024.05.24 release
+          vcpkgGitCommitId: ${{ env.VCPKG_COMMIT }}
           runVcpkgInstall: true
           doNotCache: false
         if: matrix.os == 'windows-latest'