.buildkite: add hypervisor to special_keys

[russell-islam]

Without this fix, yml is generated wrongly like below:

steps:

• label: coverage-x86_64
• command: pytest $(find . -type f -name "test_coverage.py")
• retry: automatic: false
• agents:
  • os: linux platform: x86_64.metal
  • hypervisor: mshv plugins:
• docker#v3.8.0: image: rustvmm/dev:v16
• always-pull: true privileged: true
• timeout_in_minu**tes: 5
• hypervisor: mshv // This is now right**

Signed-off-by: Muminul Islam muislam@microsoft.com

Summary of the PR

Please summarize here why the changes in this PR are needed.

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• [x] All commits in this PR are signed (with git commit -s), and the commit message has max 60 characters for the summary and max 75 characters for each description line.
• [x] All added/changed functionality has a corresponding unit/integration test.
• [x] All added/changed public-facing functionality has entries in the "Upcoming Release" section of CHANGELOG.md (if no such section exists, please create one).
• [x] Any newly added unsafe code is properly documented.