Fix Jenkins to only run branch tests on develop, not any branch with develop in name

[rok-cesnovar]

Description

It seems that Jenkins starts tests on any branch with develop in its name. For #1712 I named the branch bugfix/failing_develop and that has now started the PR tests as well as push to branch tests. The latter are not needed and should only be used for the actual develop & master branches.

Links to tests
https://jenkins.mc-stan.org/job/Math%20Pipeline/job/PR-1707/10/
https://jenkins.mc-stan.org/job/Math%20Pipeline/job/bugfix%252Ffailing_develop/1/

@serban-nicusor-toptal feel free to stop the latter tests to not waste resources on the branch tests.

Current Version:

v3.1.0

[serban-nicusor-toptal]

Hey Rok, I'm currently out of the country. Will be back on Monday and take care of all the issues. Thanks!

[rok-cesnovar]

Yeah, no worries, whenever you have time. Enjoy the time off. Just saw this issue and wrote it down before I forget :)

[serban-nicusor-toptal]

This has been fixed.

The issue was on Jenkins side and was fixed through UI.
When matching for branches the regex was .*(PR-\d+|develop|master) which would match anything_develop so it has been changed to ^(PR-\d+|develop|master)$