[VFS] Point the zarchive submodule at the parallel-writer fork [skip ci]

Collapses the stock-API build-fix detour into a single change: the zarchive
submodule now tracks xenios-jp/ZArchive (carrying the optional parallel
block-compression writer), and zar_converter.cc keeps the parallel API it has
had since it was added. The resulting tree is identical to the already-published
2.0.1-build-9730 release, so CI is skipped.

.gitmodules

@@ -72,7 +72,7 @@
 	url = https://github.com/facebook/zstd.git
 [submodule "third_party/zarchive"]
 	path = third_party/zarchive
-	url = https://github.com/exzap/ZArchive.git
+	url = https://github.com/xenios-jp/ZArchive.git
 [submodule "third_party/zlib-ng"]
 	path = third_party/zlib-ng
 	url = https://github.com/zlib-ng/zlib-ng.git