ci: run the Rust and Miri workflows only for main pushes and PRs

[zommiommy]

rust.yml and miri.yml declared empty branches: filters under both push and pull_request, so every push to a PR branch triggered both the push and the pull_request event (and tag pushes triggered extra runs too), duplicating every Rust CI and Miri check.

This scopes both workflows to the main branch, matching the existing llp.yml, so a pull request runs each check exactly once (via pull_request) and only pushes to main run the push-event checks.

on:
  push:
    branches: ["main"]
  pull_request:
    branches: ["main"]

Follow-up to #171 (the empty filters predate it; this was noticed on that PR's duplicated runs). llp.yml already used this pattern and was unaffected; build_artifact.yml triggers only on release and is untouched. This PR itself demonstrates the fix — Rust CI and Miri each trigger a single pull_request run.