vagrant: update freebsd box to 12.1

The 11.4 box showed some strange behaviour in the fuse tests and
also some network related issues.

The 12.1 box is also used by master branch and considering 12.1
just went out of support, guess it is "old enough" for 1.1-maint
and for building borg binaries.

Vagrantfile

@@ -438,7 +438,7 @@ Vagrant.configure(2) do |config|
 
   # BSD
   config.vm.define "freebsd64" do |b|
-    b.vm.box = "freebsd/FreeBSD-11.4-STABLE"
+    b.vm.box = "freebsd121-64"
     b.vm.provider :virtualbox do |v|
       v.memory = 1024 + $wmem
     end