Auto merge of #25495 - tuncer:ci-media-dummy, r=jdm

CI: build Linux amd64 --media-stack=dummy

<!-- Please describe your changes on the following line: -->
Include `--media-stack=dummy` in CI, as discussed in https://github.com/servo/servo/pull/24829.

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because it's build infra

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etc/taskcluster/decision_task.py

@@ -210,6 +210,7 @@ def linux_tidy_unit():
             python3 ./mach build --dev --features layout-2020
             python3 ./mach build --dev --libsimpleservo
             python3 ./mach build --dev -p servo-gst-plugin
+            python3 ./mach build --dev --media-stack=dummy
             python3 ./mach test-tidy --no-progress --self-test
 
             ./etc/memory_reports_over_time.py --test