sshwatcher: Don't set host name.

I guess setting host name unconditionally was a bad idea. Try to do
something non-intrusive like `uname -a`.

BUG=None
TEST=go run sshwatcher.go
  # run against a non-writable-root system.

Change-Id: I828d7ceb9d6a70b5150046bca8ce1721eed71b08
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/3201538
Tested-by: Junichi Uekawa <uekawa@chromium.org>
Reviewed-by: Satoshi Niwa <niwa@chromium.org>
Commit-Queue: Junichi Uekawa <uekawa@chromium.org>