unsafeDiscardReferences

Adds a new boolean structured attribute
`outputChecks.<output>.unsafeDiscardReferences` which disables scanning
an output for runtime references.

    __structuredAttrs = true;
    outputChecks.out.unsafeDiscardReferences = true;

This is useful when creating filesystem images containing their own embedded Nix
store: they are self-contained blobs of data with no runtime dependencies.

Setting this attribute requires the experimental feature
`discard-references` to be enabled.

tests/check-refs.nix

@@ -67,4 +67,13 @@ rec {
     disallowedReferences = [test5];
   };
 
+  test11 = makeTest 11 {
+    __structuredAttrs = true;
+    outputChecks.out = {
+      unsafeDiscardReferences = true;
+      allowedReferences = [];
+    };
+    buildCommand = ''echo ${dep} > "''${outputs[out]}"'';
+  };
+
 }