Fix logic for determining when to simulate GitHub Actions environment

[sebastianrath]

This PR enables fixes a few quirks where mimicking a GitHub environment didn't correctly work, if the graph was started from a browser session. This also changed the way which env vars are respected from which source.

GITHUB_TOKEN

1. opts.OverrideSecrets["GITHUB_TOKEN"] (highest)
2. opts.OverrideEnv["GITHUB_TOKEN"]
3. os.Getenv("GITHUB_TOKEN")
4. os.Getenv("INPUT_TOKEN") (lowest)

GITHUB_ACTIONS

1. opts.OverrideEnv["GITHUB_ACTIONS"]
2. os.Getenv("GITHUB_ACTIONS")
3. Entry node is core/gh-start@v1 (implicit)

GITHUB_WORKSPACE

1. opts.OverrideEnv["GITHUB_WORKSPACE"]
2. os.Getenv("GITHUB_WORKSPACE")

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Mimic GitHub Environment Detection

So when running outside of a real GitHub Actions runner, the system checks if it needs to simulate a GitHub environment. This happens when two things are true.

• (1) isGitHubActions is true if either defined in the config panel of the app editor, or defined via .env or shell var (ignored if defined in a config file).
• (2) All of these environment variables are not set (because its unlikely that a user sets all of these):
  • GITHUB_RUN_ID
  • RUNNER_TEMP
  • GITHUB_API_URL
  • GITHUB_RETENTION_DAYS