Auto merge of #29800 - mrobinson:only-check-gstreamer-install-for-builds, r=mukilan

Only check the GStreamer installation when building

Instead of always checking whether GStreamer is installed in mach's `build_env`, only do this when actually building. Also, use the instance variable to find features and look for the "media-gstreamer" feature instead of looking for !"media-dummy."

Fixes #29797.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #29797
- [x] These changes do not require tests because they are changes to the build scripts.

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