Switch tests from heroku/buildpacks:20 to heroku/builder:20 (#38)

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and has been replaced by the newly added `heroku/builder:20` for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).

The smoke tests in the `heroku/builder` GitHub repo are still testing
against all image variants (until such time as we make the legacy images
error with an EOL message), so we still have test coverage against them.

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

GUS-W-14186015.

.github/workflows/ci.yml

@@ -45,7 +45,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        builder: ["builder:22", "buildpacks:20"]
+        builder: ["builder:22", "builder:20"]
     env:
       INTEGRATION_TEST_CNB_BUILDER: heroku/${{ matrix.builder }}
     steps: