add-determinism: Fix meta.mainProgram, add compat symlink

The program has been renamed to add-det in 0.7.0, and upstream
recommends distros add a compatibility symlink. [1] The symlink approach
has been taken by Arch Linux. [2]

Correct meta.mainProgram to point to add-det, and add this symlink as
was recommended.

[1]: https://github.com/keszybz/add-determinism/releases/tag/v0.7.0
[2]: https://gitlab.archlinux.org/archlinux/packaging/packages/add-determinism/-/blob/5be4e2122665d4eb207239b82ec73ea1107784e2/PKGBUILD

pkgs/by-name/ad/add-determinism/package.nix

@@ -28,6 +28,10 @@ rustPlatform.buildRustPackage rec {
     ln -s ${./Cargo.lock} Cargo.lock
   '';
 
+  postInstall = ''
+    ln -s add-det $out/bin/add-determinism
+  '';
+
   doCheck = !stdenv.hostPlatform.isDarwin; # it seems to be running forever on darwin
 
   nativeBuildInputs = [
@@ -47,6 +51,6 @@ rustPlatform.buildRustPackage rec {
       sharzy
     ];
     platforms = lib.platforms.all;
-    mainProgram = "add-determinism";
+    mainProgram = "add-det";
   };
 }