Auto merge of #784 - aneeshusa:remove-global-env-vars, r=jdm
Auto merge of #784 - aneeshusa:remove-global-env-vars, r=jdm

Remove global CI environment variables

These variables are being moved to the servo/servo repo.
Keep the BUILD_MACHINE variable as it relies on Buildbot interpolation,
and cannot be moved into servo/servo.

Helps with #770; depends on servo/servo#19882.

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servo/saltfs,servo/saltfs