Add a more interesting test

In this one, we don't just output an existing derivation as is, but
modify it first.

tests/dyn-drv/recursive-mod-json.nix

@@ -0,0 +1,33 @@
+with import ./config.nix;
+
+let innerName = "foo"; in
+
+mkDerivation rec {
+  name = "${innerName}.drv";
+  SHELL = shell;
+
+  requiredSystemFeatures = [ "recursive-nix" ];
+
+  drv = builtins.unsafeDiscardOutputDependency (import ./text-hashed-output.nix).root.drvPath;
+
+  buildCommand = ''
+    export NIX_CONFIG='experimental-features = nix-command ca-derivations'
+
+    PATH=${builtins.getEnv "EXTRA_PATH"}:$PATH
+
+    # JSON of pre-existing drv
+    nix derivation show $drv | jq .[] > drv0.json
+
+    # Fix name
+    jq < drv0.json '.name = "${innerName}"' > drv1.json
+
+    # Extend `buildCommand`
+    jq < drv1.json '.env.buildCommand += "echo \"I am alive!\" >> $out/hello\n"' > drv0.json
+
+    # Used as our output
+    cp $(nix derivation add < drv0.json) $out
+  '';
+  __contentAddressed = true;
+  outputHashMode = "text";
+  outputHashAlgo = "sha256";
+}