Issue
A build is marked as pending even though it has already failed. As a result, the dashboard shows that the build is still running.
Resolution
A stuck build can be cancelled manually by running the following:
heroku plugins:install heroku-builds
heroku builds:cancel BUILD_UUID -a APP_NAME
To get the build UUID, run the command heroku builds -a APP_NAME and use the ID value for BUILD_UUID in the above command.