Update actions/cache action to v6

Author: renovate[bot]

.github/workflows/test.yaml

@@ -35,7 +35,7 @@ jobs:
 
     - name: Restore cached HOOMD-blue build
       id: cache
-      uses: actions/cache/restore@caa296126883cff596d87d8935842f9db880ef25 # v5.1.0
+      uses: actions/cache/restore@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
       with:
         path: install
         key: hoomd-blue-${{ steps.hoomd.outputs.sha }}-py313
@@ -75,7 +75,7 @@ jobs:
 
     - name: Cache HOOMD-blue build
       if: steps.cache.outputs.cache-hit != 'true'
-      uses: actions/cache/save@caa296126883cff596d87d8935842f9db880ef25 # v5.1.0
+      uses: actions/cache/save@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
       with:
         path: install
         key: hoomd-blue-${{ steps.hoomd.outputs.sha }}