Docs-only PRs from forks still run staging env in CI

[conorsch]

Description

CI shouldn't run the staging tests when a PR is prefixed with docs-. At least in the case of PRs from forks, it seems to be doing so.

Steps to Reproduce

1. Take a gander at added instructions on using libvirt Vagrant provider in Ubuntu & Debian #3670, and confirm the PR branch begins with docs-.
2. Inspect the associated CI build for that PR and confirm it did in fact run the entire staging scenario.

Expected Behavior

CI does not run staging env when branch name for PR begins with docs-.

Actual Behavior

On at least one fork, CI did run the staging env, despite the fact that the branch name for the PR began with docs-.

Comments

It would be helpful to have a verbose wrapper script to make inspecting CI state easier. The script could print informative messages such as "Detected branch name 'docs-libvirt-setup', skipping staging run...".