Update Package.swift to use GitHub Action env var

[stephencelis]

This way things don't behave strangely on other environments that set CI=1.

[rcarver]

@stephencelis I think this will be problematic in my build environment of self-hosted GitHub actions.

[stephencelis]

@rcarver Can you point your branch and verify? We can definitely set a custom env var if necessary but it'd be nice to rely on GitHub's env vars if possible.

[rcarver]

From debugging previously I have a dump of the env, which includes GITHUB_ACTION

GITHUB_ACTION_REF=
GITHUB_ACTION_REPOSITORY=
GITHUB_ACTION=__run_2
GITHUB_ACTIONS=true
GITHUB_ACTOR_ID=2343
GITHUB_ACTOR=rcarver

Here's a few vars that might be better?

GITHUB_JOB
GITHUB_WORKFLOW
GITHUB_REPOSITORY_OWNER=
GITHUB_REPOSITORY=

https://docs.github.com/en/actions/reference/workflows-and-actions/variables