Remove broken variable

Signed-off-by: apostasie <[email protected]>

Author: apostasie

.github/workflows/test.yml

@@ -390,7 +390,7 @@ jobs:
       - uses: actions/cache@5a3ec84eff668545956fd18022155c47e93e2684  # v4.2.3
         with:
           path: /root/.vagrant.d
-          key: vagrant-${{ matrix.box }}
+          key: vagrant
       - name: Set up vagrant
         run: |
           # from https://github.com/containerd/containerd/blob/v2.0.2/.github/workflows/ci.yml#L583-L596