Adjust installation for unknown default architecture

When the reported architecture is unknown and the provider is listed as
the default architecture, add the box without architecture information
so it is installed without architecture information on the path within
the collection.

lib/vagrant/action/builtin/box_add.rb

@@ -329,6 +329,17 @@ def add_from_metadata(url, env, expanded)
             provider_url = authed_urls[0]
           end
 
+          # The architecture name used when adding the box should be
+          # the value extracted from the metadata provider
+          arch_name = metadata_provider.architecture
+
+          # In the special case where the architecture name is "unknown" and
+          # it is listed as the default architecture, unset the architecture
+          # name so it is installed without architecture information
+          if arch_name == "unknown" && metadata_provider.default_architecture
+            arch_name = nil
+          end
+
           box_add(
             [[provider_url, metadata_provider.url]],
             metadata.name,
@@ -338,7 +349,7 @@ def add_from_metadata(url, env, expanded)
             env,
             checksum: metadata_provider.checksum,
             checksum_type: metadata_provider.checksum_type,
-            architecture: metadata_provider.architecture,
+            architecture: arch_name,
           )
         end
 

test/unit/vagrant/action/builtin/box_add_test.rb

@@ -1033,7 +1033,7 @@ def with_web_server(path, **opts)
           subject.call(env)
         end
 
-        it "adds the latest version of a box with only one provider and no unknown architecture set as default" do
+        it "adds the latest version of a box with only one provider and unknown architecture set as default" do
           box_path = iso_env.box2_file(:virtualbox)
           tf = Tempfile.new(["vagrant-box-latest-version", ".json"]).tap do |f|
             f.write(
@@ -1072,7 +1072,7 @@ def with_web_server(path, **opts)
             expect(name).to eq("foo/bar")
             expect(version).to eq("0.7")
             expect(opts[:metadata_url]).to eq("file://#{tf.path}")
-            expect(opts[:architecture]).to eq("unknown")
+            expect(opts[:architecture]).to be_nil
             true
           }.and_return(box)