overrideFor method
Makes moor_ffi use the open function when running on the specified
os. This can be used to override the loading behavior on some platforms.
To override that behavior on all platforms, consider using
overrideForAll.
This method must be called before opening any database.
When using the asynchronous API over isolates, open must be a top-
level function or a static method.
Implementation
void overrideFor(OperatingSystem os, OpenLibrary open) {
_overriddenPlatforms[os] = open;
}