Use respond_to

app/views/ci/variables/_content.html.haml

 %h4.prepend-top-0
   Secret variables
   = link_to icon('question-circle'), help_page_path('ci/variables/README', anchor: 'secret-variables'), target: '_blank'
-- if @variable.is_a?(Ci::VariablePresenter) && @project.feature_available?(:variable_environment_scope)
+- if @variable.respond_to?(:environment_scope) && @project.feature_available?(:variable_environment_scope)
   %p
     These variables will be set to environment by the runner, and could be protected by exposing only to protected branches or tags, or some particular environments.
 - else

app/views/ci/variables/_form.html.haml

@@ -7,7 +7,7 @@
   .form-group
     = f.label :value, "Value", class: "label-light"
     = f.text_area :value, class: "form-control", placeholder: @variable.placeholder
-  - if @variable.is_a?(Ci::VariablePresenter)
+  - if @variable.respond_to?(:environment_scope)
     = render 'projects/variables/ee/environment_scope', f: f
   .form-group
     .checkbox

app/views/ci/variables/_table.html.haml

@@ -9,7 +9,7 @@
       %th Key
       %th Value
       %th Protected
-      - if @variable.is_a?(Ci::VariablePresenter) && @project.feature_available?(:variable_environment_scope)
+      - if @variable.respond_to?(:environment_scope) && @project.feature_available?(:variable_environment_scope)
         %th Environment scope
       %th
     %tbody
@@ -19,7 +19,7 @@
             %td.variable-key= variable.key
             %td.variable-value{ "data-value" => variable.value }******
             %td.variable-protected= Gitlab::Utils.boolean_to_yes_no(variable.protected)
-            - if @variable.is_a?(Ci::VariablePresenter) && @project.feature_available?(:variable_environment_scope)
+            - if @variable.respond_to?(:environment_scope) && @project.feature_available?(:variable_environment_scope)
               %td.variable-environment-scope= variable.environment_scope
             %td.variable-menu
               = link_to variable.edit_path, class: "btn btn-transparent btn-variable-edit" do