Add gilgamesh NixOS config and spigot-server package

nix/packages/.gitignore

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+spigot-1.20.1.jar

nix/packages/README.md

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+# Packages
+
+## `spigot-server`
+
+In order to build `spigot-server.nix` you'll need a copy of the Spigot JAR.
+For licensing reasons I won't check it into Git.
+Just drop `spigot-${version}.jar` into this folder and change the `version = ` line in `spigot-server.nix` accordingly.
+
+## TODO
+
+- Make some more stuff in `spigot-server.nix` configurable

nix/packages/spigot-server.nix

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+{
+  makeWrapper,
+  stdenv,
+  # Runtime Dependencies
+  jre
+}:
+let
+  # Copied from some forum thread without much thought.
+  # Let's see if they work out.
+  javaFlags = "-Xmx5G -XX:+UseG1GC -XX:+UnlockExperimentalVMOptions -XX:MaxGCPauseMillis=50 -XX:+DisableExplicitGC -XX:TargetSurvivorRatio=90 -XX:G1NewSizePercent=50 -XX:G1MaxNewSizePercent=80 -XX:InitiatingHeapOccupancyPercent=10 -XX:G1MixedGCLiveThresholdPercent=50";
+in stdenv.mkDerivation rec {
+  pname = "spigot-server";
+  version = "1.20.1";
+  src = ./spigot-${version}.jar;
+
+  nativeBuildInputs = [ makeWrapper ];
+
+  buildCommand = ''
+    install -Dm644 $src $out/lib/spigot-${version}.jar
+
+    mkdir -p $out/bin
+    makeWrapper ${jre}/bin/java $out/bin/spigot-server \
+      --argv0 spigot-server \
+      --add-flags "${javaFlags}" \
+      --add-flags "-jar $out/lib/spigot-${version}.jar"
+  '';
+}