ci: run on pull requests from branches of this repository

[rubas-agent]

CI runs on every pull request from a branch of this repository, not only on pull requests that rubas opens.

Before: each job in ci.yml has

if: github.event_name == 'push' || github.event.pull_request.user.login == github.repository_owner

Problem

• Agents open pull requests as rubas-agent[bot], so every CI job skips on them. deps: audit with mix hex.audit and bump mint to 1.11.0 #75 shows three skipped jobs and no test run.
• The matrix job skips before it expands, so it reports OTP ${{ matrix.otp }} / Elixir ${{ matrix.elixir }} and never the real check names.

After

if: github.event_name == 'push' || github.event.pull_request.head.repo.full_name == github.repository

A pull request from a fork still skips every job. Only accounts that can push to this repository get CI, which keeps what the old rule protected.

Details

• The owner rule came in with 635c9a8 ("harden CI"), to keep CI off pull requests from other people.
• The workflow of a pull_request event comes from the pull request, so this PR runs its own CI with the new rule.
• deps: audit with mix hex.audit and bump mint to 1.11.0 #75 (the switch to mix hex.audit) waits for this PR.