[6.8] Use kibana-ci-proxy-cache for chrome and gecko drivers (… (#41633)

* Use kibana-ci-proxy-cache for chrome and gecko drivers (#41581)

* [ci] download chromedriver and geckodriver through a caching proxy

* restore chromedriver and geckodriver caching in bootstrap cache

# Conflicts:
#	.ci/packer_cache.sh
#	src/dev/ci_setup/setup.sh

* remove some extra changes

.ci/packer_cache.sh

@@ -21,6 +21,5 @@ tar -cf "$HOME/.kibana/bootstrap_cache/master.tar" \
   x-pack/node_modules \
   x-pack/plugins/*/node_modules \
   x-pack/plugins/reporting/.chromium \
-  x-pack/plugins/reporting/.phantom \
   test/plugin_functional/plugins/*/node_modules \
   .es;

src/dev/ci_setup/setup.sh

@@ -115,8 +115,9 @@ yarnGlobalDir="$(yarn global bin)"
 export PATH="$PATH:$yarnGlobalDir"
 hash -r
 
-# use a proxy to fetch geckodriver asset
-export GECKODRIVER_CDNURL="https://us-central1-elastic-kibana-184716.cloudfunctions.net/geckodriver_cache"
+# use a proxy to fetch chromedriver/geckodriver asset
+export GECKODRIVER_CDNURL="https://us-central1-elastic-kibana-184716.cloudfunctions.net/kibana-ci-proxy-cache"
+export CHROMEDRIVER_CDNURL="https://us-central1-elastic-kibana-184716.cloudfunctions.net/kibana-ci-proxy-cache"
 
 ###
 ### install dependencies