Closes #13203.

coder templates push currently only reads the template tar from stdin if the --directory flag is set to -. This is kind of awkward, and as seen in the attached issue, easy to miss. We can already check if the command is being piped to, so this is just a minor QOL improvement.

This breaks one valid albeit odd workflow, where a confirmation is piped into the command instead of using --yes/-y:

e.g:

echo 'yes' | coder templates push

would no longer work as expected.

Also updates isTTY to isTTYIn, since the name suggests it could check either stdin or stdout, but it only checks stdin, and isTTYOut already exists.