.buildkite: add hypervisor to special_keys

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_minutes: 5
  hypervisor: mshv // This is now right

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

.buildkite/autogenerate_pipeline.py

@@ -254,7 +254,7 @@ class BuildkiteStep:
         # forwarding the key, values without any change.
         # We need to filter for keys that have special meaning and which we
         # don't want to re-add.
-        special_keys = ['conditional', 'docker_plugin', 'platform', 'test_name', 'queue']
+        special_keys = ['conditional', 'docker_plugin', 'platform', 'test_name', 'queue', 'hypervisor']
         additional_keys = {k: v for k, v in input.items() if not (k in self.step_config) and
                            not(k in special_keys)}
         if additional_keys: