Adding gitSha as a step kwarg and env var

[zachncst]

To be able to find the pull request for intermittent issues, we need to add the gitSha into the build steps so the PR can be queried from github. More information here: #597. In support of: https://github.com/servo/servo/wiki/Tracking-intermittent-failures-over-time-project

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This change is 

[highfive]

Thanks for the pull request, and welcome! The Servo team is excited to review your changes, and you should hear from @aneeshusa (or someone else) soon.

[aneeshusa]

Thanks for the PR! Now that I think about it some more, getting the PR can be done entirely from mach. mach can call git rev-parse HEAD to get the current commit (since the full git history is available during Buildbot builds), then hit the GitHub API you found to find the relevant PR number. I'd like to keep as much code in servo/servo as possible, so I'm going to close this in favor of that approach.