gst-env: Ignore SIGINT when using the fish shell

gst-env: Ignore SIGINT when using the fish shell

After discussion with fish upstream it looks like it will take some
work to fix this issue.

https://github.com/fish-shell/fish-shell/pull/6426#issuecomment-567174105

In the meantime, this only happens when there's no command running in
the terminal, and in that case the shell just ignores it anyway. So
just do that in `gst-env.py`.

Fixes https://gitlab.freedesktop.org/gstreamer/gst-build/issues/18