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.

That issue has now been resolved:
buildpacks/samples#170

So we can switch back to the latest image version 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/