Use local instead of instance variable in partial

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7617#note_18762989

app/views/notify/ci/builds/_build.text.erb

-Build #<%= build.id %> ( <%= pipeline_build_url(@pipeline, build) %> )
+Build #<%= build.id %> ( <%= pipeline_build_url(pipeline, build) %> )