Use WindowsFileSystem for bootstrapping on Windows as well.

--
MOS_MIGRATED_REVID=113664915
diff --git a/src/main/java/com/google/devtools/build/lib/runtime/BlazeRuntime.java b/src/main/java/com/google/devtools/build/lib/runtime/BlazeRuntime.java
index 6c04c28..9cd7a56 100644
--- a/src/main/java/com/google/devtools/build/lib/runtime/BlazeRuntime.java
+++ b/src/main/java/com/google/devtools/build/lib/runtime/BlazeRuntime.java
@@ -990,7 +990,7 @@
   private static FileSystem fileSystemImplementation() {
     if ("0".equals(System.getProperty("io.bazel.UnixFileSystem"))) {
       // Ignore UnixFileSystem, to be used for bootstrapping.
-      return new JavaIoFileSystem();
+      return OS.getCurrent() == OS.WINDOWS ? new WindowsFileSystem() : new JavaIoFileSystem();
     }
     // The JNI-based UnixFileSystem is faster, but on Windows it is not available.
     return OS.getCurrent() == OS.WINDOWS ? new WindowsFileSystem() : new UnixFileSystem();