ci: Use a smaller Linux runner for migration checks (#64240)

## Summary

Move `check_postgres_and_protobuf_migrations` from the existing 16-vCPU
/ 32-GiB Linux runner to the existing 8-vCPU / 16-GiB runner. The job
handles Git history and Buf compatibility, lint, and formatting checks;
its commands, triggers, and coverage remain unchanged.

This reduces its concurrent CPU and memory allocation by half. Namespace
observations from September 11–14 covered 224 runs, with a typical job
duration of 37 seconds and maximum observed RAM around 1.5 GiB. The p95
of per-run peak CPU usage was about 7.7 cores, so CI runtime on the
smaller profile should be checked rather than assuming identical
performance.

Updated the Rust workflow generator and regenerated the YAML. No other
jobs are changed.

## Validation

- `cargo xtask workflows`
- `cargo fmt -p xtask -- --check`
- `git diff --check`
- Full diff reviewed: two files, only the runner selection changed.
- Actual migration check on the smaller runner awaits CI.

Release Notes:

- N/A

.github/workflows/run_tests.yml

@@ -846,7 +846,7 @@ jobs:
     needs:
     - orchestrate
     if: needs.orchestrate.outputs.run_tests == 'true'
-    runs-on: namespace-profile-16x32-ubuntu-2204
+    runs-on: namespace-profile-8x16-ubuntu-2204
     env:
       GIT_AUTHOR_NAME: Protobuf Action
       GIT_AUTHOR_EMAIL: ci@zed.dev