docs: [CI-23943]: Document hyphenated output variable behavior with act 0.2.89

[imaditya55]

…ct 0.2.89

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Description

Adds a note to the GitHub Action step docs (section: "Output variables from GitHub Actions steps") documenting a behavior change in the github-actions plugin images based on act v0.2.89 or later.
With act v0.2.89, the env-file parser (godotenv) rejects variable names containing characters outside [A-Za-z0-9_] and fails the entire step. The plugin now skips such names (e.g. hyphenated outputs like cache-hit and node-version from actions/setup-node) when writing act's env file. These outputs remain available as Harness output variables and can be referenced normally in later steps/stages.
This documents the change so support/customers have a reference if they notice hyphenated variables missing from the action's runtime environment after upgrading the plugin image.
Related plugin PR: drone-plugins/github-actions#32

Jira

https://harness.atlassian.net/browse/CI-24652

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