Install dependencies for the WPT update task

Fix https://github.com/servo/servo/issues/25169

etc/taskcluster/decision_task.py

@@ -583,7 +583,7 @@ def update_wpt():
     return (
         linux_task("WPT update")
         .with_treeherder("Linux x64", "WPT update")
-        .with_dockerfile(dockerfile_path("run"))
+        .with_dockerfile(dockerfile_path("wpt-update"))
         .with_features("taskclusterProxy")
         .with_scopes("secrets:get:project/servo/wpt-sync")
         .with_index_and_artifacts_expire_in(log_artifacts_expire_in)

etc/taskcluster/docker/run.dockerfile

@@ -12,4 +12,4 @@ RUN apt-get install -qy --no-install-recommends \
     libunwind8 \
     #
     # Compiling shaders in OSMesa:
-    llvm \
+    llvm

etc/taskcluster/docker/wpt-update.dockerfile

@@ -0,0 +1,5 @@
+% include run.dockerfile
+
+RUN apt-get install -qy --no-install-recommends \
+    python3 \
+    jq