cmd: Forward stdin.

[shlevy]

Fixes #95.

[MikeGoldsmith]

Hi @shlevy thank you for your contribution.

I don't see a problem with this change, but would like to get @maplebed's opinion as they are more familiar with buildevents and to to check it would not have unexpected consequences.

[maplebed]

Thanks for the bump, @MikeGoldsmith. Under most conditions I can think of where buildevents is used there's nothing on stdin, so this will be a noop change for those cases.

I would love to see a real world example of how this change will be used - @shlevy maybe you could drop a few examples in either #95 or this PR? I don't think we should block on waiting for examples, but I do think it will help give context to the change if it does cause problems down the line and will ensure any future fixes take into account the intent of the change.

[shlevy]

Here's an example from a diff where I added instrumentation to our CI:

- kubectl exec -i svc/scarf-redis -- redis-cli < ./redis_dummydata.txt
+ buildevents cmd --quiet $TRACE_ID $STEP_SPAN_ID  load-redis-data -- kubectl exec -i svc/scarf-redis -- redis-cli < ./redis_dummydata.txt