Don't fail on inaccessible paths

When a library path (from `/etc/ld.so.conf` or `LD_LIBRARY_PATH`) is not accessible for the current user, `nixglhost` fails. For example: $ LD_LIBRARY_PATH=/root nixglhost ls ... File "/nix/store/ch7nppfsp3zpdyimralb1zc5h6dpz9sx-nix-gl-host-0.1/bin/nixglhost", line 302, in resolve_libraries for fname in os.listdir(path): ^^^^^^^^^^^^^^^^ PermissionError: [Errno 13] Permission denied: '/root' This patch fixes that by printing a warning and continuing: $ LD_LIBRARY_PATH=/root nixglhost ls WARNING: [Errno 13] Permission denied: '/root' README.md ...