fix double updating bug

[jdx]

running heroku update still causes the blocking update to run. This is happening because when the CLI is updated, the update lock file is cleared to force an autoupdate and reexec of the command. Generally this is good, because it will make sure that everything is updated using the new CLI. The problem is when the command is actually update, it will force a blocking update, then run the command which is to update.

This just checks before running a blocking update if the command is to update. If so, there is no need to do the blocking update step.

before:

$ heroku update master
Updating Heroku v4 CLI to 4.27.25-3b727e5 (master)... done
heroku-cli: Adding dependencies... done
Updating plugins... no plugins to update.
Updating plugins... no plugins to update.

after:

$ heroku update dev
Updating Heroku v4 CLI to 4.27.25-b97ac29 (dev)... done
heroku-cli: Adding dependencies... done
heroku-cli: Updating plugins... no plugins to update.

[jdx]

ready for review @ransombriggs

[ransombriggs]

@dickeyxxx lgtm