ci: run miri nightly instead of on every push and PR

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

The miri workflow currently runs on every push to main and on every pull request, each gated by a paths-ignore ignore list. Miri runs are slow, and running them on every PR adds latency to the build queue for little benefit, since memory-safety regressions in native code are rare.

It would be better to run these tests nightly.

What changes are included in this PR?

Replaces the on: triggers in .github/workflows/miri.yml:

• Removes the push trigger and its paths-ignore list
• Removes the pull_request trigger and its paths-ignore list
• Adds a schedule trigger running nightly at 04:00 UTC (cron: '0 4 * * *')
• Keeps workflow_dispatch for manual runs

The miri job itself is unchanged. The cron time does not collide with the existing scheduled workflows (codeql 16 4 * * 1, stale 30 1 * * *).

How are these changes tested?

This is a CI configuration change. The workflow remains manually triggerable via workflow_dispatch to verify it runs, and the nightly schedule will exercise it on the normal cadence.

[comphead]

Thanks @andygrove that makes a lot of sense. We would need to check miri workflow manually