Let workflow 1 run on pull requests from forks

[marvinm2]

Workflow 1 runs on pull_request_target, and two of its jobs check out refs/pull/N/head. actions/checkout v6 refuses that, so both fail at their first step for any pull request from a fork. Same-repo pull requests are unaffected, which is why it has not come up before.

Changes:

• get-gpml fetches the GPML over the API instead of checking it out. The file is read as data, and the generators already run from base code via the gpml-file artifact, so no fork code is executed. GH_REPO is added because without a checkout gh has no remote to infer the repository from.
• update-pr-desc never used its checkout. It reads job outputs and runs gh pr edit, so the step is removed.
• The git push in Get branch name needed a working tree. Nothing consumes the branch-name output and workflow 3a checks out main, so it is removed as well.

I did not use allow-unsafe-pr-checkout. This workflow holds the deploy keys, and checking out fork code there is the risk the refusal exists to prevent.

Tested on a fork of the sandbox repo: run 30906919228, all ten jobs green on a real cross-repository pull request. A workflow_dispatch re-run does not test this, since it runs in the base context with a full token.