web: serveFile: also serve a CSP putting served HTML in its own origin

src/lib/Hydra/Controller/Build.pm

@@ -236,6 +236,9 @@ sub serveFile {
     }
 
     elsif ($ls->{type} eq "regular") {
+        # Have the hosted data considered its own origin to avoid being a giant
+        # XSS hole.
+        $c->response->header('Content-Security-Policy' => 'sandbox allow-scripts');
 
         $c->stash->{'plain'} = { data => grab(cmd => ["nix", "--experimental-features", "nix-command",
                                                       "store", "cat", "--store", getStoreUri(), "$path"]) };