Return after single arch push

[djeremiah]

When running crossplane xpkg push with a package built with a single architecture, the push command mistakenly falls through to the code block that adds an index to the manifest list. If done with a package built on anything other than linux/amd64, this confuses the package manager later, which will error out with a message like the following:

0s Warning ParsePackage configurationrevision/platform-ref-apigateway-187041da383f cannot initialize parser backend: failed to fetch package from remote: no child with platform linux/amd64 in index index.docker.io/steve/platform-ref-apigateway:v0.1.0

This PR adds a return statement inside the len(c.PackageFiles) == 1 case.

Fixes #4980

[negz]

Looks good! Can you update the description with a little more info on the bug this fixes, it’ll help with release notes. We’ll need to ship a patch release to fix this.

[github-actions]

Successfully created backport PR for release-1.14:

• [Backport release-1.14] Return after single arch push #4978