ci: force checkout the filter example (#2575)
ci: force checkout the filter example (#2575)

A later stage in the script modifies WORKSPACE in the filter-example directory;
if this directory persists between builds, this may cause the checkout of
a particular commit to fail due to modified local workspace.

Risk Level: Low
Testing: Manually tested in broken scenario

Signed-off-by: Greg Greenway <f58b4fce410ab27c1d5cc4afd1f2a190e9aa65c3@apple.com>
envoyproxy/envoy,istio/envoy,lyft/envoy,eklitzke/envoy,lyft/envoy,eklitzke/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,lizan/envoy,istio/envoy,istio/envoy,dnoe/envoy,envoyproxy/envoy,jrajahalme/envoy,dnoe/envoy,envoyproxy/envoy-wasm,craffert0/envoy,eklitzke/envoy,dnoe/envoy,envoyproxy/envoy-wasm,istio/envoy,lizan/envoy,craffert0/envoy,sebrandon1/envoy,lyft/envoy,envoyproxy/envoy-wasm,istio/envoy,dnoe/envoy,jrajahalme/envoy,jrajahalme/envoy,eklitzke/envoy,jrajahalme/envoy,sebrandon1/envoy,istio/envoy,envoyproxy/envoy-wasm,lyft/envoy,envoyproxy/envoy,lizan/envoy,sebrandon1/envoy,envoyproxy/envoy,sebrandon1/envoy,craffert0/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy-wasm,jrajahalme/envoy,istio/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,sebrandon1/envoy,lizan/envoy,dnoe/envoy,jrajahalme/envoy,envoyproxy/envoy,craffert0/envoy,lizan/envoy,envoyproxy/envoy,lizan/envoy,lyft/envoy,eklitzke/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,eklitzke/envoy