Fix creation of ffprobe-test.nut on remote targets.

Fix creation of ffprobe-test.nut on remote targets.

This is not sufficient to run "make fate-ffprobe" on a remote system:
The ffprobe output contains the relative path to the testfile, it is
necessary to run the test from the build directory.
One solution is to use a script like the following as --target-exec:
ssh target "cd /remote/build/directory; $(printf "%q " "$@")"