Suppress warnings for System.getProperty("line.separator") (apple#432)

`class System` gets initialized at build time through `native-image`
and added to the heap. We initialize everything at build time via the
`--initialize-at-build-time=` flag.

pkl-core/src/main/java/org/pkl/core/util/IoUtils.java

@@ -175,6 +175,7 @@ public static Path getDefaultModuleCacheDir() {
   }
 
   // not stored to avoid build-time initialization by native-image
+  @SuppressWarnings("SystemGetProperty")
   public static String getLineSeparator() {
     return System.getProperty("line.separator");
   }