fix(core_dl): fix zip extraction with extractsrc across devices and non-empty dirs

Using mv to move extracted directories fails in two cases:
- Cross-device moves (e.g. tmp and serverfiles on different Docker volumes)
- Target directory already exists and is non-empty (update scenario)

Replace find+mv with cp -a which handles both cases by copying recursively
and merging into the destination. Also replace the hardcoded 'Xonotic'
temp_extractdir with ${extractsrc} to be generic.

lgsm/modules/core_dl.sh

@@ -267,9 +267,9 @@ fn_dl_extract() {
 		fi
 	elif [ "${mime}" == "application/zip" ]; then
 		if [ -n "${extractsrc}" ]; then
-			temp_extractdir="${tmpdir}/Xonotic"
+			temp_extractdir="${tmpdir}/${extractsrc}"
 			extractcmd=$(unzip -qo "${local_filedir}/${local_filename}" "${extractsrc}/*" -d "${temp_extractdir}")
-			find "${temp_extractdir}/${extractsrc}" -mindepth 1 -maxdepth 1 -exec mv -t "${extractdest}" {} +
+			cp -a "${temp_extractdir}/${extractsrc}/." "${extractdest}/"
 			rm -rf "${temp_extractdir}"
 		else
 			extractcmd=$(unzip -qo -d "${extractdest}" "${local_filedir}/${local_filename}")