Fix thanos make-local pipeline

ci/prow/test.local pipeline is currently broken due to the build04 cluster addressed to it in the buildfarm being a bit slow and making github.com/thanos-io/thanos/pkg/store go over the default 900s.

panic: test timed out after 15m0s1027running tests:1028	TestTSDBStoreSeries (4m3s)1029	TestTSDBStoreSeries/1SeriesWith10000000Samples (2m58s)1030 

Extending it makes the test pass

 ok github.com/thanos-io/thanos/pkg/store 984.344s

We'll be addressing this alongside a follow-up issue to address this with an env var in upstream Thanos.