Switch back to using heroku/procfile from the builder in tests (#713)

The `heroku/builder:22` builder image will now only ever contain one
version of the `heroku/procfile` CNB, after:
heroku/buildpacks-jvm#608
heroku/buildpacks-nodejs#696

As such, we no longer need to use a manual Docker Hub URL to avoid the
"multiple versions of this buildpack exist in the builder image" error,
and instead can use the Procfile CNB directly from the builder image.

This does mean the version is now unpinned, however, it's unlikely to
cause test failures in practice, and at least we'll be testing the
version of procfile actually used in the builder image (for things like
the container starting tests).

libcnb-test/tests/integration_test.rs

@@ -17,11 +17,7 @@ use std::path::PathBuf;
 use std::time::Duration;
 use std::{env, fs, thread};
 
-// We're referencing the procfile buildpack via Docker URL to pin the version for the tests. This also
-// prevents issues when the builder contains multiple heroku/procfile versions. We don't use CNB
-// registry URLs since, as of August 2022, pack fails when another pack instance is resolving such
-// an URL in parallel.
-const PROCFILE_URL: &str = "docker://docker.io/heroku/procfile-cnb:2.0.1";
+const PROCFILE_URL: &str = "heroku/procfile";
 const TEST_PORT: u16 = 12345;
 
 #[test]