Revert "nixos/tests/elk: remove OSS ELK-7 test"

This reverts commit 35db6bc5543d352ea7e54375ba12acd70033a7f9.

nixos/tests/elk.nix

···
           )
     '';
   }) { inherit pkgs system; };
-in pkgs.lib.mapAttrs mkElkTest ({
+in pkgs.lib.mapAttrs mkElkTest {
   ELK-6 =
     if enableUnfree
     then {
···
       journalbeat   = pkgs.journalbeat6;
       metricbeat    = pkgs.metricbeat6;
     };
-} // pkgs.lib.optionalAttrs enableUnfree {
-    ELK-7 =
-      {
-        elasticsearch = pkgs.elasticsearch7;
-        logstash      = pkgs.logstash7;
-        kibana        = pkgs.kibana7;
-        journalbeat   = pkgs.journalbeat7;
-        metricbeat    = pkgs.metricbeat7;
-      };
-})
+  ELK-7 =
+    if enableUnfree
+    then {
+      elasticsearch = pkgs.elasticsearch7;
+      logstash      = pkgs.logstash7;
+      kibana        = pkgs.kibana7;
+      journalbeat   = pkgs.journalbeat7;
+      metricbeat    = pkgs.metricbeat7;
+    }
+    else {
+      elasticsearch = pkgs.elasticsearch7-oss;
+      logstash      = pkgs.logstash7-oss;
+      kibana        = pkgs.kibana7-oss;
+      journalbeat   = pkgs.journalbeat7;
+      metricbeat    = pkgs.metricbeat7;
+    };
+}