scripts/install-nix-from-closure: only show progress if a terminal is used

While the progress dots during the copying of the store work fine on a
normal terminal, those look pretty off if the script is run inside a
provisioning script of e.g. `vagrant` or `packer` where `stderr` and
`stdout` are captured:

    default: .
    default: ..
    default: .
    default: .
    default: .

To work around this, the script checks with `-t 0` if it's
running on an actual terminal and doesn't show the progress if that's not
the case.

0 files changed, 0 insertions, 0 deletions