Use the system NSS CA trust library

Quoting the commit:

    [xulrunner] Use the system NSS CA trust library instead of the bundled copy

    With the addition of p11-kit, we want to unify the CA trust stores
    across the platform. p11-kit integrates with NSS by replacing
    libnssckbi, the CA trust library, with a binary compatible version that
    accesses the p11-kit store.

    This is implemented in the system-wide nss and p11-kit packages by replacing a
    subpackage that contains this library. For this browser version, so long
    as we can rely on the ABI being compatible between browser nss, system nss,
    and p11-kit, we can simply symlink to the system library and get the
    same results.

Requires https://git.merproject.org/mer-core/nss/merge_requests/3 (for the nss-ckbi subpackage).