[EFL] Make the Performance bot also build WebKit.
https://bugs.webkit.org/show_bug.cgi?id=109273

Reviewed by Philippe Normand.

There has not been much benefit in fetching binaries compiled by
another bot; it makes the build bot take 6 to 7 minutes to upload
the binaries to build.webkit.org, we need to manually override
LD_LIBRARY_PATH to find the compiled libraries and still have
other path-related problems.

Since we have enough horsepower in the perf bot, let's experiment
with building WebKit there instead.

* BuildSlaveSupport/build.webkit.org-config/config.json:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@142249 268f45cc-cd09-0410-ab3c-d52691b4dbfc