docs: Salty documentation workflow executes fork-controlled Python build hooks with deployment secrets

The Salty documentation repository builds and deploys the docs.saltbox.dev website. Prior to commit 649c9aea2daef1168eeb0415b44e65dbbde9f220, .github/workflows/trigger-pr-build.yml records an untrusted fork commit in the pr_info artifact, and the secret-bearing workflow_run job in .github/workflows/build-pr.yml checks out that commit before running pip install -r requirements.txt. A fork pull request can therefore supply attacker-authored setup.py or PEP 517 build hooks whose installation code executes in the privileged base-repository workflow with CF_API_TOKEN, GH_TOKEN, MKDOCS_GIT_COMMITTERS_APIKEY, and a write-capable GITHUB_TOKEN, allowing secret disclosure and follow-on compromise. The vulnerability is not caused by the repository-fork guard, and the demonstrated path requires attacker control of fork content rather than merely the artifact metadata. This vulnerability is fixed with commit 649c9aea2daef1168eeb0415b44e65dbbde9f220.