Windows Jenkins build failure

I believe that ```SLAVE-07FA0C31A``` is a new build machine. It seems that perhaps JRuby is not installed in the expected location.

This may be related to the Windows 2012 build referenced on https://github.com/elastic/logstash/issues/7767
```
Started by upstream project "elastic+logstash+pull-request" build number 534
originally caused by:
GitHub pull request #7798 of commit db913b73e81087a09d4f3c804b2f8ee4f99a53fe, no merge conflicts.
[EnvInject] - Loading node environment variables.
[EnvInject] - Preparing an environment for the build.
[EnvInject] - Keeping Jenkins system variables.
[EnvInject] - Keeping Jenkins build variables.
[EnvInject] - Injecting as environment variables the properties content
HOME=C:\Users\jenkins

[EnvInject] - Variables injected successfully.
[EnvInject] - Injecting contributions.
Building remotely on SLAVE-07FA0C31A (virtual swarm win32nt windows) in workspace C:\Users\jenkins\workspace\elastic+logstash+pull-request+multijob-windows-compatibility
Wiping out workspace first.
Cloning the remote Git repository
Cloning repository git@github.com:elastic/logstash.git
> git init C:\Users\jenkins\workspace\elastic+logstash+pull-request+multijob-windows-compatibility # timeout=10
ERROR: Reference path does not exist: /var/lib/jenkins/.git-references/logstash.git
Fetching upstream changes from git@github.com:elastic/logstash.git
> git --version # timeout=10
using GIT_SSH to set credentials master node jenkins user ~/.ssh
> git fetch --tags --progress git@github.com:elastic/logstash.git +refs/heads/*:refs/remotes/origin/*
> git config remote.origin.url git@github.com:elastic/logstash.git # timeout=10
> git config --add remote.origin.fetch +refs/heads/*:refs/remotes/origin/* # timeout=10
> git config remote.origin.url git@github.com:elastic/logstash.git # timeout=10
Fetching upstream changes from git@github.com:elastic/logstash.git
using GIT_SSH to set credentials master node jenkins user ~/.ssh
> git fetch --tags --progress git@github.com:elastic/logstash.git +refs/pull/*:refs/remotes/origin/pr/*
> git rev-parse "db913b73e81087a09d4f3c804b2f8ee4f99a53fe^{commit}" # timeout=10
Checking out Revision db913b73e81087a09d4f3c804b2f8ee4f99a53fe (detached)
> git config core.sparsecheckout # timeout=10
> git checkout -f db913b73e81087a09d4f3c804b2f8ee4f99a53fe
> git rev-list 2f8820fc52352fde57e47e7328c6a00161fca446 # timeout=10
[elastic+logstash+pull-request+multijob-windows-compatibility] $ cmd /c call C:\Users\JENKIN~1.SLA\AppData\Local\Temp\jenkins876995373950640783.bat

C:\Users\jenkins\workspace\elastic+logstash+pull-request+multijob-windows-compatibility>set JRUBYSRCDIR=C:\Progra~1\jruby\src

C:\Users\jenkins\workspace\elastic+logstash+pull-request+multijob-windows-compatibility>ci\ci_test.bat
"Variable JRUBYSRCDIR must be declared with a valid directory. Aborting.."
Build step 'Execute Windows batch command' marked build as failure
Sending e-mails to: infra-root+build@elastic.co
Finished: FAILURE
```

Start with ci/ci_test.bat and the Jenkins output for build machine SLAVE-07FA0C31A, especially the JRUBYSRCDIR assignment and directory validation. Verify the expected JRuby location on the Windows worker and compare it with the related Windows 2012 issue; done means the Windows compatibility build completes successfully.

Written by the indexing model from the issue text.