update GitHub actions to latest version

the build didn't work anymore with the old versions since GH de-released
`cache@v2`.

Author: rursprung

.github/workflows/rust.yml

@@ -14,8 +14,8 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
-      - uses: actions/cache@v2
+      - uses: actions/checkout@v4
+      - uses: actions/cache@v4
         with:
           path: |
             ~/.cargo/registry