Split out os-specific cask software version checks

Library/Homebrew/diagnostic.rb

@@ -910,22 +910,6 @@ module Homebrew
 
       def check_cask_software_versions
         add_info "Homebrew Version", HOMEBREW_VERSION
-        add_info "macOS", MacOS.full_version
-        add_info "SIP", begin
-          csrutil = "/usr/bin/csrutil"
-          if File.executable?(csrutil)
-            Open3.capture2(csrutil, "status")
-                 .first
-                 .gsub("This is an unsupported configuration, likely to break in " \
-                       "the future and leave your machine in an unknown state.", "")
-                 .gsub("System Integrity Protection status: ", "")
-                 .delete("\t.")
-                 .capitalize
-                 .strip
-          else
-            "N/A"
-          end
-        end
 
         nil
       end

Library/Homebrew/extend/os/linux/diagnostic.rb

@@ -189,6 +189,13 @@ module OS
             are prone to breaking when GCC is updated. You should `brew reinstall` these formulae:
           EOS
         end
+
+        def check_cask_software_versions
+          super
+          add_info "Linux", OS::Linux.os_version
+
+          nil
+        end
       end
     end
   end

Library/Homebrew/extend/os/mac/diagnostic.rb

@@ -496,6 +496,28 @@ module OS
             #{installation_instructions}
           EOS
         end
+
+        def check_cask_software_versions
+          super
+          add_info "macOS", MacOS.full_version
+          add_info "SIP", begin
+            csrutil = "/usr/bin/csrutil"
+            if File.executable?(csrutil)
+              Open3.capture2(csrutil, "status")
+                   .first
+                   .gsub("This is an unsupported configuration, likely to break in " \
+                         "the future and leave your machine in an unknown state.", "")
+                   .gsub("System Integrity Protection status: ", "")
+                   .delete("\t.")
+                   .capitalize
+                   .strip
+            else
+              "N/A"
+            end
+          end
+
+          nil
+        end
       end
     end
   end