Don't use jimfs when testing DocMigratorTest

`copyToRecursively` does not work correctly when copying from Windows
paths to jimfs.

Author: bioball

pkl-doc/src/test/kotlin/org/pkl/doc/DocMigratorTest.kt

@@ -17,10 +17,7 @@
 
 package org.pkl.doc
 
-import com.google.common.jimfs.Configuration
-import com.google.common.jimfs.Jimfs
 import java.io.OutputStream
-import java.nio.file.FileSystem
 import java.nio.file.Files
 import java.nio.file.Path
 import kotlin.io.path.*
@@ -38,8 +35,6 @@ import org.pkl.core.util.IoUtils
 
 class DocMigratorTest {
   companion object {
-    private val tempFileSystem: FileSystem by lazy { Jimfs.newFileSystem(Configuration.unix()) }
-
     private val projectDir = FileTestUtils.rootProjectDir.resolve("pkl-doc")
 
     private val inputDir: Path by lazy {
@@ -53,7 +48,7 @@ class DocMigratorTest {
     }
 
     private val actualOutputDir: Path by lazy {
-      tempFileSystem.getPath("/work/output").createDirectories()
+      Files.createTempDirectory("docMigratorTest")
     }
 
     private val actualOutputFiles: List<Path> by lazy { actualOutputDir.listFilesRecursively() }