Suppress `scp` output from `stdout` in `//labgrid/run`

When running //labgrid/run with bazel run (via labgrid_binary), the scp output is being piped to stdout.

Example output of running echo hello:

mktemp                         100%  758KB  61.1MB/s   00:00    
mkdir                          100%  792KB  62.9MB/s   00:00    
mv                             100%  947KB  69.7MB/s   00:00    
rm                             100%  812KB  63.4MB/s   00:00    
printenv                       100%  765KB  61.4MB/s   00:00    
env                            100%  773KB  63.3MB/s   00:00    
hello