Restrict Ubuntu.1404.Arm32.Open from running against PRs

[echesakov]

Disable Linux/arm32 testing for now to avoid "Test Linux arm32" jobs being cancelled in each PR
(e.g. #22255 (comment))

The problem with this queue that it can't handle all the work items coming from PRs, push triggered builds and JitStress scheduled builds. Re-trying build from the AzDO page are making this even worse since they newly started jobs add new work items to the queue without removing already submittted ones.

The long term solution should be Helix being able to remove scheduled jobs if a corresponing AzDO build has been cancelled. In other words, if no one expects to see the test results coming from Helix submission why bother running them? @MattGal probably knows about the issue - but I don't know when the feature is going to be implemented.

A potential short term solution could be switching to Docker based Arm32 queue, but, as far as I remember, @jashook had some objections of doing this (at least it was my impression).

I am personally fine with an alternative when one of these queues are used exclusively for PR triggered builds and others queues are being utilized to run JitStress or CI builds.

cc @dotnet/jit-contrib

[BruceForstall]

I don't recall our Jenkins CI having any capacity issues with these jobs recently. Why is AzDO stressing it more that Jenkins did? Are there more jobs running than before? Is it because Helix is using all the machines at once, and the overhead of the split jobs is much higher than running all Pri-0 jobs on a single host?

[echesakov]

@BruceForstall I think the problem is that now when you submit jobs to Helix from AzDO job your build is timeout-ed (after roughly 3 hours for pri0) if the jobs haven't been completed by that time while before Jenkins jobs were indefinitely waiting for the job to start running. When your AzDO job is timeout-ed you click this re-try button, effectively, submitting even more jobs to the queue (without removing previosly submitted ones) - you job are get cancelled again - you re-try again.