fish: ignore SIGINT and set prompt correctly

Fixes #18 (moved)

commit 675cec1e:

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

commit e13e6758:

gst-env: Set the prompt for fish to be same as bash

commit 245baadb:

gst-env: Fix shell name check
We should use `endswith`, not `in`. Else we'll match paths like:

`/home/arbash/.local/bin/fish` as a bash shell, not a fish shell.

Assigned to @meh because he's been investigating the fish SIGINT issue upstream. Mathieu, does this look like a decent workaround till the upstream bug is fixed?