Fix `hk log` under heroku-agent; don't mutate `http.DefaultTransport`

Unfortunately, having `HEROKU_AGENT_SOCK` set currently breaks `hk log` 
because I overwrite the default `Dial` function. When `hk log` tries to 
retrieve the URL for its log session, it can no longer perform a normal HTTPS
call properly.

This patches that issue by making `tr` just define a new transport which it
will then use for calls to the Heroku API and PG APIs. Even in devclouds,
setting `TLSClientConfig` should only be required for calls to the API, so I
don't _think_ that this will have any negative side effects on the program.