pizauth: fix systemd feature

Although we had `systemd` included in `buildFeatures`, it didn't reach
`$out/bin/pizauth`, because upstream's Makefile used
`target/release/pizauth` as a Makefile target that `cargo build
--release` (with or without `--features=systemd`) solves.

pkgs/by-name/pi/pizauth/package.nix

@@ -27,6 +27,14 @@ rustPlatform.buildRustPackage (finalAttrs: {
   preConfigure = ''
     substituteInPlace lib/systemd/user/pizauth.service \
       --replace-fail /usr/bin/ ''${!outputBin}/bin/
+    # Upstream's Makefile uses target/release/pizauth as a Makefile target that
+    # the `install` target depends upon. Nixpkgs' cargoBuildHook defaults to
+    # using the explicit `--target @rustcTargetSpec@` flag, so that the
+    # executable always ends up in
+    # `target/${stdenv.hostPlatform.rust.rustcTargetSpec}/release`. To make the
+    # Makefile not run cargo build again, we use this substitution.
+    substituteInPlace Makefile \
+      --replace-fail target/release target/${stdenv.hostPlatform.rust.rustcTargetSpec}/release
   '';
 
   postInstall = ''