add a utility for making a nix release app

README.md

@@ -54,6 +54,7 @@ A nix flake for use with Haskell, for Least Authority.
 		  ];
 	  };
 	  packages = hslib.packages {};
+	  apps.release = hslib.apps.release { };
 	});
 }
 ```

lib.nix

@@ -76,6 +76,24 @@ rec {
   };
 
   apps = {
+    # Upload a candidate or (with `--publish`) publish a release.
+    release = { }: {
+      type = "app";
+      program = "${
+        pkgs.writeShellApplication {
+          name = "release";
+          runtimeInputs = with pkgs; [cabal-install];
+          text = ''
+            set -x
+            sdist=$(cabal sdist | tail -n 1)
+            haddocks=$(cabal haddock --haddock-for-hackage | tail -n 1)
+            cabal upload "$@" "$sdist"
+            cabal upload "$@" --documentation "$haddocks"
+          '';
+        }
+      }/bin/release";
+    };
+
     # Create a program that runs hlint.
     hlint = {
       # An argv to pass to hlint in addition to any arguments passed on the