MIRI jobs consume a lot of GH runner action time

### Describe the bug

The Infra Report (needs a login, not sure how many people can see this)
https://infra-reports.apache.org/#ghactions&project=arrow&hours=168&limit=15&group=name

Shows that our MIRI job is consuming 16% of the total GH action usage for the Arrow project -- this is across all language implementations.

This seems like a large amount of time / cost to devote to a job compared to its relative benefit.

### To Reproduce

Do nothing

### Expected behavior

It would be nice to reduce the cost of running the MIRI tests

Perhaps we could change the job so it:
1. does not run by default on all PRs / pushes (only run on commits to main)
2. can be run manually on particular PRs if needed

### Additional context

_No response_

Start by locating the GitHub Actions workflow that defines the MIRI job and inspect its current triggers and runtime. Compare the available trigger options with the issue's proposed main-branch and manual-run behavior, then verify that the resulting workflow no longer runs unnecessarily on all PRs or pushes while remaining runnable when needed.

Written by the indexing model from the issue text.