Switch tests from heroku/buildpacks:20 to heroku/builder:20 (#693)

Since the `heroku/buildpacks:20` image is about to be deprecated,
and has been replaced by the newly added `heroku/builder:20` for
those that need to use a Heroku-20 based builder.

The build and run images used by `heroku/builder:20` are the same as
those used by `heroku/buildpacks:20` - the only differences between the
builders are the included buildpacks (which is somewhat irrelevant for
these integration tests, since the buildpack under test will be injected instead).

See:
heroku/cnb-builder-images#394
https://salesforce.quip.com/0JtbAYiWZYk6

GUS-W-14186015.

examples/ruby-sample/tests/integration_test.rs

@@ -19,7 +19,7 @@ use std::{fs, io};
 #[test]
 #[ignore = "integration test"]
 fn basic() {
-    let config = BuildConfig::new("heroku/buildpacks:20", "tests/fixtures/simple-ruby-app");
+    let config = BuildConfig::new("heroku/builder:20", "tests/fixtures/simple-ruby-app");
 
     TestRunner::default().build(&config, |context| {
         assert_contains!(context.pack_stdout, "---> Ruby Buildpack");
@@ -60,7 +60,7 @@ fn basic() {
 #[ignore = "integration test"]
 fn missing_gemfile_lock() {
     TestRunner::default().build(
-        BuildConfig::new("heroku/buildpacks:20", "tests/fixtures/simple-ruby-app")
+        BuildConfig::new("heroku/builder:20", "tests/fixtures/simple-ruby-app")
             .app_dir_preprocessor(|path| fs::remove_file(path.join("Gemfile.lock")).unwrap())
             .expected_pack_result(PackResult::Failure),
         |context| {