Rename the property io.bazel.UnixFileSystem to io.bazel.EnableJni because that's what it does.

In particular, it affects Windows.

--
MOS_MIGRATED_REVID=126293339
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 0a922fc..f11b51a 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
@@ -885,7 +885,7 @@
   }
 
   private static FileSystem fileSystemImplementation() {
-    if ("0".equals(System.getProperty("io.bazel.UnixFileSystem"))) {
+    if ("0".equals(System.getProperty("io.bazel.EnableJni"))) {
       // Ignore UnixFileSystem, to be used for bootstrapping.
       return OS.getCurrent() == OS.WINDOWS ? new WindowsFileSystem() : new JavaIoFileSystem();
     }