Use an absolute path when loading a native library (#7208)

Fix #7206

This is an workaround of #7207

I don't think we want following this route, but since Mill is already
bootstrapped with a Mill version affected by this issue, we need to fix
it temporarily.

Tested manually with `mill -i __.prepareOffline`.

Pull request: #7208

Author: lefou

runner/launcher/src/mill/launcher/JLineNativeLoader.scala

@@ -109,7 +109,7 @@ object JLineNativeLoader {
     System.setProperty("library.jline.path", loader.millJLineNativeDir.toString)
     System.setProperty("library.jline.name", loader.millJLineNativeLibLocation.last)
 
-    System.load(loader.millJLineNativeLibLocation.toString)
+    System.load(loader.millJLineNativeLibLocation.toNIO.toAbsolutePath().toString())
 
     val cls = classOf[org.jline.nativ.JLineNativeLoader]
     val fld = cls.getDeclaredField("loaded")