Move away from Travis CI to GitHub Actions

tools/ci/install_byond.sh

@@ -0,0 +1,21 @@
+#!/bin/bash
+set -euo pipefail
+
+source _build_dependencies.sh
+
+if [ -d "$HOME/BYOND/byond/bin" ] && grep -Fxq "${BYOND_MAJOR}.${BYOND_MINOR}" $HOME/BYOND/version.txt;
+then
+  echo "Using cached directory."
+else
+  echo "Setting up BYOND."
+  rm -rf "$HOME/BYOND"
+  mkdir -p "$HOME/BYOND"
+  cd "$HOME/BYOND"
+  curl "http://www.byond.com/download/build/${BYOND_MAJOR}/${BYOND_MAJOR}.${BYOND_MINOR}_byond_linux.zip" -o byond.zip
+  unzip byond.zip
+  rm byond.zip
+  cd byond
+  make here
+  echo "$BYOND_MAJOR.$BYOND_MINOR" > "$HOME/BYOND/version.txt"
+  cd ~/
+fi