Explicitly set github repo in instance presenter spec (#24036)

spec/presenters/instance_presenter_spec.rb

@@ -89,8 +89,28 @@ describe InstancePresenter do
   end
 
   describe '#source_url' do
-    it 'returns "https://github.com/mastodon/mastodon"' do
-      expect(instance_presenter.source_url).to eq('https://github.com/mastodon/mastodon')
+    context 'with the GITHUB_REPOSITORY env variable set' do
+      around do |example|
+        ClimateControl.modify GITHUB_REPOSITORY: 'other/repo' do
+          example.run
+        end
+      end
+
+      it 'uses the env variable to build a repo URL' do
+        expect(instance_presenter.source_url).to eq('https://github.com/other/repo')
+      end
+    end
+
+    context 'without the GITHUB_REPOSITORY env variable set' do
+      around do |example|
+        ClimateControl.modify GITHUB_REPOSITORY: nil do
+          example.run
+        end
+      end
+
+      it 'defaults to the core mastodon repo URL' do
+        expect(instance_presenter.source_url).to eq('https://github.com/mastodon/mastodon')
+      end
     end
   end