Suppress some compilation messages in non-interactive R sessions

[wlandau]

Submission Checklist

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

Summary

Omit compilation messages "Compiling Stan program..." and "Model executable is up to date!" for non-interactive R sessions. Fixed #429.

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):

Eli Lilly and Company

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)

[wlandau]

Unit tests tests passed on my local machine in both an interactive and non-interactive session. Looks like there are some issues installing MPI in the GitHub Actions workflows.

[rok-cesnovar]

@wlandau Thanks! Will take a look shortly. Seems there is an unrelated Ubuntu package installation issue, will restart tests until it goes away :)

Err:9 http://azure.archive.ubuntu.com/ubuntu focal-updates/main amd64 libxnvctrl0 amd64 440.82-0ubuntu0.20.04.1
  404  Not Found [IP: 52.252.75.106 80]

[rok-cesnovar]

@wlandau please merge master in whenever you have the chance. The CI fix was pushed there.

[rok-cesnovar]

LGTM!

Thanks @wlandau!