Use built-in concurrency feature in Github Actions

[WardBrian]

This uses GitHub's concurrency feature rather than the n1hility/cancel-previous-runs third-party step we were using before

Summary

This will make it so only one workflow per "group" is allowed to run. Groups are defined by a key given as ${{ github.workflow }}-${{ github.head_ref || github.run_id }}. The docs provide more information, but the end result is that this will only let one copy of this workflow run in PRs (the key will be something like main.yml-PR####), but will not cancel runs on branches (where instead the key will be something like main.yml-SomeUniqueNumber)

Tests

Side Effects

Release notes

Checklist

• Math issue #(issue number)

• Copyright holder: Simons Foundation

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the license the submitted work under the following licenses:
  - Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
  - Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

• the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • dependencies checks pass, (make test-math-dependencies)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

• the code is written in idiomatic C++ and changes are documented in the doxygen

• the new changes are tested

[rok-cesnovar]

Assuming tests pass, this is good to go. Thanks!

Is this a fairly new feature of GHA?

[WardBrian]

Relatively new. The action we used before started recommending using this on Feb 1: n1hility/cancel-previous-runs@73dabc9