filesystem::read_symlink and weakly_canonical fails in parallel access on Windows likely due to wrong access flag

This is described well in [this stack overflow article](https://stackoverflow.com/questions/44393241/boostfilesystemrelative-cannot-access-the-file-because-it-is-being-used-by#) and has examples and a sample fix.
Two processes accessing a file via link (read-only) get a file sharing violation, because read-only is not passed to the underlying file access call (CreateFileW parameter)