Remove OpenGL Core from HDRP Tests on Linux

[jenniferd-unity]

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Purpose of this PR

The Linux edit mode tests are failing at the moment on Global Settings tests.
When debugging on a VM machine, I noticed that OpenGL Core was the used Graphics API which is not compatible with HDRP, Vulkan should be changed. This PR enforces Vulkan usage for the Editor (-force-vulkan commandline is only forcing the Player/Play mode).
More info on slack here

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Testing status

On bokken, I tested with both Editor open and with command line to run HDRP_Tests. They all were green.

[github-actions]

Hi! This comment will help you figure out which jobs to run before merging your PR. The suggestions are dynamic based on what files you have changed.
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HDRP
/.yamato%252Fall-hdrp.yml%2523PR_HDRP_2021.2

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