vagrant: Install vagrant-libvirt as system plugin

pkgs/development/tools/vagrant/default.nix

···
 
     mkdir -p "$out/vagrant-plugins/plugins.d"
     echo '{}' > "$out/vagrant-plugins/plugins.json"
+  '' +
+  lib.optionalString withLibvirt ''
+    substitute ${./vagrant-libvirt.json.in} $out/vagrant-plugins/plugins.d/vagrant-libvirt.json \
+      --subst-var-by ruby_version ${ruby.version} \
+      --subst-var-by vagrant_version ${version}
   '';
 
   installCheckPhase = ''

pkgs/development/tools/vagrant/vagrant-libvirt.json.in

···
+{
+  "vagrant-libvirt": {
+    "ruby_version":"@ruby_version@",
+    "vagrant_version":"@vagrant_version@",
+    "gem_version":"",
+    "require":"",
+    "sources":[]
+  }
+}