Tests: Fix sporadic failures to find docker

[WardBrian]

Summary

See runs like https://jenkins.flatironinstitute.org/blue/organizations/jenkins/Stan%2FMath/detail/PR-2832/20/pipeline

This started after #2826. I believe it's a simple fix to request that the nodes are only linux ones.

Checklist

• Math issue #(issue number)

• Copyright holder: (fill in copyright holder information)

  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

[WardBrian]

Hmm, https://jenkins.flatironinstitute.org/blue/organizations/jenkins/Stan%2FMath/detail/PR-2836/1/pipeline failed despite running on a Linux node as far as I can tell. Not sure why Docker wouldn't be recognized

[WardBrian]

Okay, I have added the label "linux && docker" to those stages and it seems like the problem went away. It was only happening occasionally before though, so it might take a few runs to be sure.

At any rate, this definitely should not make anything worse

[serban-nicusor-toptal]

LGTM! Thanks.