This gets rid of bashWrap() and of running bash -s passing it a script
on stdin. Also get rid of most of the runBash*() helpers.
In particular, `go run hack/e2e.go -build` will now work when it needs
to ask whether it is OK to download a large docker image, it used to
fail since stdin was not available for the bash script using the `read`
command.
Tested by running a full `hack/e2e-test.sh` run including the build
stage without the docker image loaded, which used to fail before this
change.
Eric Paris