Revert "svm: don't use Path in startup code where stuff the FileSystemProviders might not be initialized"

This reverts commit b0c29d6.

Author: zapster

substratevm/src/com.oracle.svm.core.posix/src/com/oracle/svm/core/posix/attach/PosixAttachApiSupport.java

@@ -26,7 +26,7 @@
 
 package com.oracle.svm.core.posix.attach;
 
-import java.nio.file.FileSystems;
+import java.nio.file.Paths;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.locks.ReentrantLock;
 
@@ -181,7 +181,7 @@ private String getSocketFilePath() {
         if (cachedSocketFilePath == null) {
             long pid = ProcessHandle.current().pid();
             String tempDir = Target_jdk_internal_vm_VMSupport.getVMTemporaryDirectory();
-            cachedSocketFilePath = tempDir + FileSystems.getDefault().getSeparator() + ".java_pid" + pid;
+            cachedSocketFilePath = Paths.get(tempDir, ".java_pid" + pid).toString();
         }
         return cachedSocketFilePath;
     }