Disable building Envoy filter example in CI

[tbarrella]

See istio/istio#30484

[bianpengyuan]

@tbarrella do you intent to target this to master branch and then cherry-pick this to release branches later?

[tbarrella]

I think so since it's failing in both 1.7 and 1.8 (do we have an automated way of creating cherry picks for this repo?)

[bianpengyuan]

I don't think auto cherry-pick is set up for this repo. We don't really use master branch of this repo so I think it is fine to just make the change to release branches.

[tbarrella]

Opened #296 and #297

[istio-testing]

@tbarrella: The following tests failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
test-tsan_envoy	994e879	link	/test test-tsan_envoy
test-asan_envoy	994e879	link	/test test-asan_envoy
test-release_envoy	994e879	link	/test test-release_envoy

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[PiotrSikora]

Yeah, master branch is not even kept up to date in this repo, please only use release branches.

Could you also send this cherry-pick for 1.9?