Bump actions/cache from 4 to 5

Bumps [actions/cache](https://github.com/actions/cache) from 4 to 5.
- [Release notes](https://github.com/actions/cache/releases)
- [Changelog](https://github.com/actions/cache/blob/main/RELEASES.md)
- [Commits](actions/cache@v4...v5)

---
updated-dependencies:
- dependency-name: actions/cache
  dependency-version: '5'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/rust.yml

@@ -37,7 +37,7 @@ jobs:
       - uses: dtolnay/rust-toolchain@nightly
 
       - name: Cache cargo registry + build artifacts
-        uses: actions/cache@v4
+        uses: actions/cache@v5
         with:
           path: |
             ~/.cargo/bin

.github/workflows/vdf-client-hw.yml

@@ -120,15 +120,15 @@ jobs:
       - name: Cache Boost on Windows
         if: matrix.os == 'windows-latest'
         id: cache-boost
-        uses: actions/cache/restore@v4
+        uses: actions/cache/restore@v5
         with:
           path: C:\local\boost_*
           key: windows-boost-msvc-14.3-v1
 
       - name: Cache mpir checkout on Windows
         if: matrix.os == 'windows-latest'
         id: cache-mpir
-        uses: actions/cache@v4
+        uses: actions/cache@v5
         with:
           path: mpir_gc_x64
           key: windows-mpir-gc-x64-v1
@@ -173,7 +173,7 @@ jobs:
 
       - name: Save Boost cache on Windows
         if: matrix.os == 'windows-latest' && steps.cache-boost.outputs.cache-hit != 'true'
-        uses: actions/cache/save@v4
+        uses: actions/cache/save@v5
         with:
           path: C:\local\boost_*
           key: ${{ steps.cache-boost.outputs.cache-primary-key }}