Deletes that follow symbolic links: plugin, build script and runtime #227

Kotlin's deleteRecursively follows symbolic links, so deleting a directory that contains a link to outside deletes the outside target too (found in pypackpack #96 / toolchain #93, where call-site tests proved it on production paths).

Sites (listed by the PMM build supporter):

• Gradle plugin: IosPythonHome.kt:222 home; PythonArtifactBindingsTask.kt:104 outputDirectory, :107 thunkRoot; PythonStubsTask.kt:89 destination.
• python-multiplatform/build.gradle.kts:633 iosExtractDir, :1687 resultsDir, :1825 resultsRoot.
• Runtime: PythonBootstrap.kt:150 payloadDir, :228 stdlibDir (Android); PythonPayload.desktop.kt:257 root (desktop).
  Safe already: PythonHomeStaging.kt:473, WasmBrowserRuntimeStaging.kt:75 (Gradle fs.delete).

Done when every site uses a delete that does not follow links, with regression tests (a link to an outside directory survives) observed red first.