ci: increase memory size of vmactions workflow

By default, the VM gets 1024MiB. https://github.com/vmactions/freebsd-vm/issues/28 suggests to increase it.

.github/workflows/vmactions.yml

@@ -15,6 +15,7 @@ jobs:
     - name: Build
       uses: vmactions/freebsd-vm@v0
       with:
+        mem: 8192
         usesh: true
         prepare: |
           pkg install -y \