chore: update to actions/cache@v4 (#52)

This is to fix this audit warning:
    Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: actions/cache@v3. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

Author: Eric Swanson

.github/workflows/test.yml

@@ -17,7 +17,7 @@ jobs:
     steps:
       - uses: actions/checkout@master
 
-      - uses: actions/cache@v3
+      - uses: actions/cache@v4
         with:
           path: |
             ~/.cargo/registry