nix copy: falling back to dumb output on interactive terminal

[Profpatsch]

Original Motivation:

When using nix copy on CI (to push to an s3 bucket), usually the build times out after 10 minutes because nothing is printed to CI

Solution: use -v, CI will usually not be an interactive terminal

Issue:

Turning of the fancy output on interactive shells requires stderr to be redirected to a non-interactive pipe like this:

nix copy -v … 2>&1 | cat

which is a bit hard to find out.

Request:

Fall back to simple output if TERM is set to dumb (which is customary and should be the default when using terminfo (stackoverflow source).