Because we wrongly set $GST_PLUGIN_SYSTEM_PATH="" in the shell... Uploading a patch.
Created attachment 404715 [details] Patch
Comment on attachment 404715 [details] Patch LGTM
Committed r264618: <https://trac.webkit.org/changeset/264618> All reviewed patches have been landed. Closing bug and clearing flags on attachment 404715 [details].
<rdar://problem/65848057>