ci: rework Linux build scripts, allow crosscompiling with whatever is included in Ubuntu 20.04 (which is used on GitHub Actions now) repositories

scripts/gha/linux/AppRun

@@ -0,0 +1,10 @@
+#!/bin/sh
+
+if [ "$XASH3D_BASEDIR" = "" ]; then
+	export XASH3D_BASEDIR="$PWD"
+fi
+echo "Xash3D FWGS installed as AppImage."
+echo "Base directory is $XASH3D_BASEDIR. Set XASH3D_BASEDIR environment variable to override this."
+
+export XASH3D_EXTRAS_PAK1="${APPDIR}/valve/extras.pk3"
+exec $DEBUGGER "${APPDIR}/xash3d" "$@"

scripts/gha/linux/xash3d-fwgs.desktop

@@ -0,0 +1,11 @@
+[Desktop Entry]
+Categories=Game;Shooter;
+Comment=Half-Life compatible game engine
+Exec=AppRun
+Icon=xash3d-fwgs
+Keywords=first;person;shooter;multiplayer;half-life;halflife;singleplayer;
+Name=Xash3D FWGS
+# Doesn't pass validation with old desktop-file-utils
+#PrefersNonDefaultGPU=true
+Terminal=false
+Type=Application