Make test "can pivot to new rootfs recursively" work on NixOS

[quag]

The "can pivot to new rootfs recursively" test is the only test that fails due to the quirks of NixOS. Specifically, NixOS does not have /lib or /sbin directories. To fix, I've made /lib and /sbin --ro-bind-try instead of --ro-bind.

Adding --ro-bind-try /nix/store /nix-store as findmnt is located there. As this is the only use of findmnt in the tests, we could also use an alternative way to check if /usr has been mounted in the container.

As NixOS is out of step with mainstream Linux distros, I understand if bubblewrap doesn't want to support it for it's tests.

[smcv]

As NixOS is out of step with mainstream Linux distros, I understand if bubblewrap doesn't want to support it for it's tests.

If the necessary changes were much more intrusive than this then I'd be saying no, but since it's this simple, OK.