sing-box: install shell completions directly

Author: Prince213

pkgs/by-name/si/sing-box/package.nix

@@ -1,10 +1,8 @@
 {
   lib,
-  stdenv,
   buildGoModule,
   fetchFromGitHub,
   installShellFiles,
-  buildPackages,
   coreutils,
   nix-update-script,
   nixosTests,
@@ -45,21 +43,14 @@ buildGoModule (finalAttrs: {
     "-X=github.com/sagernet/sing-box/constant.Version=${finalAttrs.version}"
   ];
 
-  postInstall =
-    let
-      emulator = stdenv.hostPlatform.emulator buildPackages;
-    in
-    ''
-      installShellCompletion --cmd sing-box \
-        --bash <(${emulator} $out/bin/sing-box completion bash) \
-        --fish <(${emulator} $out/bin/sing-box completion fish) \
-        --zsh  <(${emulator} $out/bin/sing-box completion zsh )
+  postInstall = ''
+    installShellCompletion release/completions/sing-box.{bash,fish,zsh}
 
-      substituteInPlace release/config/sing-box{,@}.service \
-        --replace-fail "/usr/bin/sing-box" "$out/bin/sing-box" \
-        --replace-fail "/bin/kill" "${coreutils}/bin/kill"
-      install -Dm444 -t "$out/lib/systemd/system/" release/config/sing-box{,@}.service
-    '';
+    substituteInPlace release/config/sing-box{,@}.service \
+      --replace-fail "/usr/bin/sing-box" "$out/bin/sing-box" \
+      --replace-fail "/bin/kill" "${coreutils}/bin/kill"
+    install -Dm444 -t "$out/lib/systemd/system/" release/config/sing-box{,@}.service
+  '';
 
   passthru = {
     updateScript = nix-update-script { };