fix(internal): vendor-aware GHES detection in isGhes() to support non-GitHub servers

[MahdiBaghbani]

Introduce a minimal, explicit vendor switch to prevent isGhes() from misclassifying non GitHub platforms (e.g., Forgejo/Gitea) as GHES, This PR adds a positive allowlist for GHES aliases and preserves current behavior for github.

On self-hosted non-GitHub servers, the current heuristic (GITHUB_SERVER_URL hostname) often evaluates to enterprise sending actions down GHES specific code paths and causing failures.
A tiny, explicit environment switch avoids this without changing default behavior on GitHub

for reference when this breaks other platforms please see:
actions/upload-artifact#676
https://code.forgejo.org/actions/upload-artifact
https://code.forgejo.org/forgejo/runner/src/branch/main/RELEASE-NOTES.md#3-4-0

Although this version is able to run actions/upload-artifact@v4 and actions/download-artifact@v4, these actions will fail because it does not run against GitHub.com. A fork of those two actions with this check disabled is made available at:
https://code.forgejo.org/forgejo/upload-artifact/src/tag/v4
https://code.forgejo.org/forgejo/download-artifact/src/tag/v4

go-gitea/gitea#31256

The workaround right now is:
https://gitea.com/actions/gitea-upload-artifact/commit/81f940d004763f986ba3582c007fd842dd5cb0d7
https://code.forgejo.org/forgejo/upload-artifact/commit/16871d9e8cfcf27ff31822cac382bbb5450f1e1e