Remove /cache from nightly benchmark (#4386)

This pull requests removes any mention of the `/cache` directory from
the nightly benchmark pipeline.

The `/cache` directory was symlinked to `/tmp`, which got cleaned up
during a system update. This may not have been the right solution in the
first place.

### Issue number

None. Found after system update.

.buildkite/nightly.yml

@@ -24,8 +24,6 @@ steps:
 
   - label: 'Restore benchmark - cardano mainnet'
     command: "./.buildkite/bench-restore.sh mainnet"
-    env:
-      HOME: "/cache/cardano-wallet.home"
     timeout_in_minutes: 1200
     agents:
       system: x86_64-linux