Unpin version of the example alpine builder used in CI

In CI a `pack build` is performed using one of libcnb's example CNBs
and the upstream `cnb/sample-builder:alpine` builder, in order to test
that our static MUSL compile works with non-libc base images.

However, in #594 the version of that builder had to be pinned to
an older release, to work around a temporary incompatibility
between the latest version of the builder and the latest stable
Pack CLI.

Now that Pack CLI 0.30 has been released, the latest alpine builder
will work again.

.github/workflows/ci.yml

@@ -71,6 +71,4 @@ jobs:
         working-directory: ./examples/basics
       - name: Pack build using examples/basics
         # Uses a non-libc image to validate the static musl cross-compilation.
-        # TODO: Switch this back to using the `alpine` tag once the stable Pack CLI release supports
-        # image extensions (currently newer sample alpine images fail to build with stable Pack).
-        run: pack build example-basics --builder cnbs/sample-builder@sha256:da5ff69191919f1ff30d5e28859affff8e39f23038137c7751e24a42e919c1ab --trust-builder --buildpack packaged/x86_64-unknown-linux-musl/debug/libcnb-examples_basics --path examples/
+        run: pack build example-basics --builder cnbs/sample-builder:alpine --trust-builder --buildpack packaged/x86_64-unknown-linux-musl/debug/libcnb-examples_basics --path examples/