Bug 1685663 - Change the geckodriver index-search path. (#17370)

* Change the geckodriver index-search path.

* Run some tests.

* Add platform name to the index-search path.

* Undo testing changes.

taskcluster/ci/toolchain/gecko-derived.yml

@@ -31,7 +31,7 @@ linux64-geckodriver:
     description: "Geckodriver toolchain"
     run:
         index-search:
-            - gecko.cache.level-3.toolchains.v3.linux64-geckodriver.latest
+            - gecko.v2.mozilla-central.latest.geckodriver.linux64
 
 linux64-minidump-stackwalk:
     attributes: