github/workflows: only sync files from runner to vm for freebsd

This option was just added to the new release tag so bump it to 0.20. We
don't care about syncing files back to the runner, so only sync one way
to the VM to hopefully make one type of error related to ejecting the
hard drive rarer*.

*: cross-platform-actions/action#64

.github/workflows/build.yml

@@ -176,9 +176,10 @@ jobs:
     steps:
     - uses: actions/checkout@v3
     - name: Test in FreeBSD VM
-      uses: cross-platform-actions/action@v0.19.1
+      uses: cross-platform-actions/action@v0.20.0
       with:
         operating_system: freebsd
+        sync_files: runner-to-vm
         version: '13.2'
         run: |
             sudo pkg update