Simplify GitHub Actions

[ajschmidt8]

The Login to DockerHub step should only be performed on non-forks (i.e. the source repo), since the secrets aren't available in forks. This PR simplifies the logic to make that happen.

Closes #4.

[ajschmidt8]

In hindsight, I don't think this can be simplified too much more. The following didn't work:

• moving the secrets directly to the conditional statement (https://github.com/rapidsai/mambaforge-cuda/pull/5/commits/4aac176217623e712f501dfe4f0cc117e1d18c68): apparently the secrets object can't be used in conditional statements (src)
• determining whether it's a fork from a property on the github object (docs) - the GitHub object's top-level properties don't provide any values to know whether a given workflow is executing on a fork. Evaluating the github.event properties also doesn't work since our shared workflow is executed on two different events (i.e. pull-request and push) and therefore has two different payloads

This isn't a huge deal, so we'll just keep things as-is for now.