This is the official Heroku buildpack for installing the Salesforce CLI into your Heroku dyno. It will also install jq, a command-line tool that is like sed for JSON data, which makes it easy to interact with JSON output from the Salesforce CLI.
To use sfdx and jq, you simply need to export the appropriate paths:
export PATH="$BUILD_DIR/vendor/sfdx/cli/bin:$PATH"
export PATH="$BUILD_DIR/vendor/sfdx/jq:$PATH"The $BUILD_DIR is the path where your apps source is stored on the Heroku dyno.