Revert "Remove an exception from the git user default SSH config check"

This is a CE-only change for now

This reverts commit 2f0f62e5.

lib/system_check/app/git_user_default_ssh_config_check.rb

@@ -11,10 +11,10 @@ module SystemCheck
       ].freeze
 
       set_name 'Git user has default SSH configuration?'
-      set_skip_reason 'skipped (git user is not present / configured)'
+      set_skip_reason 'skipped (GitLab read-only, or git user is not present / configured)'
 
       def skip?
-        !home_dir || !File.directory?(home_dir)
+        Gitlab::Database.read_only? || !home_dir || !File.directory?(home_dir)
       end
 
       def check?

spec/lib/system_check/app/git_user_default_ssh_config_check_spec.rb

@@ -39,6 +39,14 @@ describe SystemCheck::App::GitUserDefaultSSHConfigCheck do
 
       it { is_expected.to eq(expected_result) }
     end
+
+    it 'skips GitLab read-only instances' do
+      stub_user
+      stub_home_dir
+      allow(Gitlab::Database).to receive(:read_only?).and_return(true)
+
+      is_expected.to be_truthy
+    end
   end
 
   describe '#check?' do