CI: Consolidate custom tarball workflow into main unit test workflow

[Copilot]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

The cmdstan-tarball-check.yaml workflow has fallen out of sync with R-CMD-check.yaml due to separate maintenance. This PR consolidates them by adding optional custom tarball support to the main workflow via workflow_dispatch.

Changes:

• R-CMD-check.yaml: Added workflow_dispatch trigger with optional tarball_url input. Installation logic now checks CMDSTAN_TEST_TARBALL_URL and uses custom tarball when provided:

  tarball_url <- Sys.getenv("CMDSTAN_TEST_TARBALL_URL")
  if (nzchar(tarball_url) && tarball_url != "latest") {
    cmdstanr::install_cmdstan(cores = 2, release_url = tarball_url)
  } else {
    cmdstanr::install_cmdstan(cores = 2)
  }

• cmdstan-tarball-check.yaml: Updated to modern r-lib actions (setup-r@v2, setup-r-dependencies@v2, check-r-package@v2). Added deprecation notice. Logic aligned with main workflow.

• .github/workflows/README.md: Documented manual workflow trigger process for custom tarball testing.

Usage: Navigate to Actions → "Unit tests" → "Run workflow", enter tarball URL (or leave empty for latest).

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
GitHub Copilot

By submitting this pull request, the copyright holder is agreeing to
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/)

Original prompt

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This section details on the original issue you should resolve

<issue_title>CI: Update custom tarball action</issue_title>
<issue_description>We use this action during the CmdStan releases but it has fallen out of date. One good way to keep this in sync is to just update your normal unit testing action to optionally accept a custom version, that way you don't need to maintain things twice.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes CI: Update custom tarball action #1130

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[jgabry]

Closing, will use my version in #1132 instead.